diff --git a/harness/temporalHelpers.js b/harness/temporalHelpers.js
index c6f190fb263..c9b7645b7f1 100644
--- a/harness/temporalHelpers.js
+++ b/harness/temporalHelpers.js
@@ -238,39 +238,6 @@ var TemporalHelpers = {
     });
   },
 
-  /*
-   * checkFractionalSecondDigitsOptionWrongType(temporalObject):
-   *
-   * Checks the string-or-number type handling of the fractionalSecondDigits
-   * option to the various types' toString() methods. temporalObject is an
-   * instance of the Temporal type under test.
-   */
-  checkFractionalSecondDigitsOptionWrongType(temporalObject) {
-    // null is not a number, and converts to the string "null", which is an invalid string value
-    assert.throws(RangeError, () => temporalObject.toString({ fractionalSecondDigits: null }), "null");
-    // Booleans are not numbers, and convert to the strings "true" or "false", which are invalid
-    assert.throws(RangeError, () => temporalObject.toString({ fractionalSecondDigits: true }), "true");
-    assert.throws(RangeError, () => temporalObject.toString({ fractionalSecondDigits: false }), "false");
-    // Symbols are not numbers and cannot convert to strings
-    assert.throws(TypeError, () => temporalObject.toString({ fractionalSecondDigits: Symbol() }), "symbol");
-    // BigInts are not numbers and convert to strings which are invalid
-    assert.throws(RangeError, () => temporalObject.toString({ fractionalSecondDigits: 2n }), "bigint");
-
-    // Objects are not numbers and prefer their toString() methods when converting to a string
-    assert.throws(RangeError, () => temporalObject.toString({ fractionalSecondDigits: {} }), "plain object");
-
-    const toStringExpected = temporalObject.toString({ fractionalSecondDigits: 'auto' });
-    const expected = [
-      "get fractionalSecondDigits.toString",
-      "call fractionalSecondDigits.toString",
-    ];
-    const actual = [];
-    const observer = TemporalHelpers.toPrimitiveObserver(actual, "auto", "fractionalSecondDigits");
-    const result = temporalObject.toString({ fractionalSecondDigits: observer });
-    assert.sameValue(result, toStringExpected, "object with toString");
-    assert.compareArray(actual, expected, "order of operations");
-  },
-
   /*
    * checkPlainDateTimeConversionFastPath(func):
    *
diff --git a/test/built-ins/Temporal/Duration/prototype/toString/fractionalseconddigits-invalid-string.js b/test/built-ins/Temporal/Duration/prototype/toString/fractionalseconddigits-invalid-string.js
index 036b977ad16..5af9938e232 100644
--- a/test/built-ins/Temporal/Duration/prototype/toString/fractionalseconddigits-invalid-string.js
+++ b/test/built-ins/Temporal/Duration/prototype/toString/fractionalseconddigits-invalid-string.js
@@ -16,6 +16,7 @@ features: [Temporal]
 
 const duration = new Temporal.Duration(1, 2, 3, 4, 5, 6, 7, 987, 650, 0);
 
-for (const fractionalSecondDigits of ["other string", "AUTO", "not-auto", "autos"]) {
-  assert.throws(RangeError, () => duration.toString({ fractionalSecondDigits }));
+for (const fractionalSecondDigits of ["other string", "AUTO", "not-auto", "autos", "auto\0"]) {
+  assert.throws(RangeError, () => duration.toString({ fractionalSecondDigits }),
+    `"${fractionalSecondDigits}" is not a valid value for fractionalSecondDigits`);
 }
diff --git a/test/built-ins/Temporal/Duration/prototype/toString/fractionalseconddigits-out-of-range.js b/test/built-ins/Temporal/Duration/prototype/toString/fractionalseconddigits-out-of-range.js
index 9a20541c19f..ab73ad69bd1 100644
--- a/test/built-ins/Temporal/Duration/prototype/toString/fractionalseconddigits-out-of-range.js
+++ b/test/built-ins/Temporal/Duration/prototype/toString/fractionalseconddigits-out-of-range.js
@@ -16,7 +16,11 @@ features: [Temporal]
 
 const duration = new Temporal.Duration(1, 2, 3, 4, 5, 6, 7, 987, 650, 0);
 
-assert.throws(RangeError, () => duration.toString({ fractionalSecondDigits: -Infinity }));
-assert.throws(RangeError, () => duration.toString({ fractionalSecondDigits: -1 }));
-assert.throws(RangeError, () => duration.toString({ fractionalSecondDigits: 10 }));
-assert.throws(RangeError, () => duration.toString({ fractionalSecondDigits: Infinity }));
+assert.throws(RangeError, () => duration.toString({ fractionalSecondDigits: -Infinity }),
+  "−∞ is out of range for fractionalSecondDigits");
+assert.throws(RangeError, () => duration.toString({ fractionalSecondDigits: -1 }),
+  "−1 is out of range for fractionalSecondDigits");
+assert.throws(RangeError, () => duration.toString({ fractionalSecondDigits: 10 }),
+  "10 is out of range for fractionalSecondDigits");
+assert.throws(RangeError, () => duration.toString({ fractionalSecondDigits: Infinity }),
+  "∞ is out of range for fractionalSecondDigits");
diff --git a/test/built-ins/Temporal/Duration/prototype/toString/fractionalseconddigits-undefined.js b/test/built-ins/Temporal/Duration/prototype/toString/fractionalseconddigits-undefined.js
index 2c3af23ae92..292b8df300a 100644
--- a/test/built-ins/Temporal/Duration/prototype/toString/fractionalseconddigits-undefined.js
+++ b/test/built-ins/Temporal/Duration/prototype/toString/fractionalseconddigits-undefined.js
@@ -16,10 +16,21 @@ info: |
 features: [Temporal]
 ---*/
 
-const duration = new Temporal.Duration(1, 2, 3, 4, 5, 6, 7, 987, 650, 0);
+const wholeSeconds = new Temporal.Duration(1, 2, 3, 4, 5, 6, 7);
+const subSeconds = new Temporal.Duration(1, 2, 3, 4, 5, 6, 7, 987, 650);
 
-const explicit = duration.toString({ fractionalSecondDigits: undefined });
-assert.sameValue(explicit, "P1Y2M3W4DT5H6M7.98765S", "default fractionalSecondDigits is auto");
+const tests = [
+  [wholeSeconds, "P1Y2M3W4DT5H6M7S"],
+  [subSeconds, "P1Y2M3W4DT5H6M7.98765S"],
+];
 
-const implicit = duration.toString({});
-assert.sameValue(implicit, "P1Y2M3W4DT5H6M7.98765S", "default fractionalSecondDigits is auto");
+for (const [duration, expected] of tests) {
+  const explicit = duration.toString({ fractionalSecondDigits: undefined });
+  assert.sameValue(explicit, expected, "default fractionalSecondDigits is auto (property present but undefined)");
+
+  const implicit = duration.toString({});
+  assert.sameValue(implicit, expected, "default fractionalSecondDigits is auto (property not present)");
+
+  const lambda = duration.toString(() => {});
+  assert.sameValue(lambda, expected, "default fractionalSecondDigits is auto (property not present, function object)");
+}
diff --git a/test/built-ins/Temporal/Duration/prototype/toString/fractionalseconddigits-wrong-type.js b/test/built-ins/Temporal/Duration/prototype/toString/fractionalseconddigits-wrong-type.js
index 143de7c3440..625daaff1da 100644
--- a/test/built-ins/Temporal/Duration/prototype/toString/fractionalseconddigits-wrong-type.js
+++ b/test/built-ins/Temporal/Duration/prototype/toString/fractionalseconddigits-wrong-type.js
@@ -22,4 +22,26 @@ features: [Temporal]
 ---*/
 
 const duration = new Temporal.Duration(1, 2, 3, 4, 5, 6, 7, 987, 650, 0);
-TemporalHelpers.checkFractionalSecondDigitsOptionWrongType(duration);
+
+assert.throws(RangeError, () => duration.toString({ fractionalSecondDigits: null }),
+  "null is not a number and converts to the string 'null' which is not valid for fractionalSecondDigits");
+assert.throws(RangeError, () => duration.toString({ fractionalSecondDigits: true }),
+  "true is not a number and converts to the string 'true' which is not valid for fractionalSecondDigits");
+assert.throws(RangeError, () => duration.toString({ fractionalSecondDigits: false }),
+  "false is not a number and converts to the string 'false' which is not valid for fractionalSecondDigits");
+assert.throws(TypeError, () => duration.toString({ fractionalSecondDigits: Symbol() }),
+  "symbols are not numbers and cannot convert to strings");
+assert.throws(RangeError, () => duration.toString({ fractionalSecondDigits: 2n }),
+  "bigints are not numbers and convert to strings which are not valid for fractionalSecondDigits");
+assert.throws(RangeError, () => duration.toString({ fractionalSecondDigits: {} }),
+  "plain objects are not numbers and convert to strings which are not valid for fractionalSecondDigits");
+
+const expected = [
+  "get fractionalSecondDigits.toString",
+  "call fractionalSecondDigits.toString",
+];
+const actual = [];
+const observer = TemporalHelpers.toPrimitiveObserver(actual, "auto", "fractionalSecondDigits");
+const result = duration.toString({ fractionalSecondDigits: observer });
+assert.sameValue(result, "P1Y2M3W4DT5H6M7.98765S", "object with toString uses toString return value");
+assert.compareArray(actual, expected, "object with toString calls toString and not valueOf");
diff --git a/test/built-ins/Temporal/Duration/prototype/toString/smallestunit-valid-units.js b/test/built-ins/Temporal/Duration/prototype/toString/smallestunit-valid-units.js
index fce836b31b1..24880177e23 100644
--- a/test/built-ins/Temporal/Duration/prototype/toString/smallestunit-valid-units.js
+++ b/test/built-ins/Temporal/Duration/prototype/toString/smallestunit-valid-units.js
@@ -9,12 +9,37 @@ features: [Temporal]
 
 const duration = new Temporal.Duration(1, 2, 3, 4, 5, 6, 7, 987, 654, 321);
 
-assert.sameValue(duration.toString({ smallestUnit: "second" }), "P1Y2M3W4DT5H6M7S");
-assert.sameValue(duration.toString({ smallestUnit: "millisecond" }), "P1Y2M3W4DT5H6M7.987S");
-assert.sameValue(duration.toString({ smallestUnit: "microsecond" }), "P1Y2M3W4DT5H6M7.987654S");
-assert.sameValue(duration.toString({ smallestUnit: "nanosecond" }), "P1Y2M3W4DT5H6M7.987654321S");
+function test(instance, expectations, description) {
+  for (const [smallestUnit, expectedResult] of expectations) {
+    assert.sameValue(instance.toString({ smallestUnit }), expectedResult,
+      `${description} with smallestUnit "${smallestUnit}"`);
+  }
+}
+
+test(
+  duration,
+  [
+    ["seconds", "P1Y2M3W4DT5H6M7S"],
+    ["milliseconds", "P1Y2M3W4DT5H6M7.987S"],
+    ["microseconds", "P1Y2M3W4DT5H6M7.987654S"],
+    ["nanoseconds", "P1Y2M3W4DT5H6M7.987654321S"],
+  ],
+  "subseconds toString"
+);
+
+test(
+  new Temporal.Duration(1, 2, 3, 4, 5, 6, 7),
+  [
+    ["seconds", "P1Y2M3W4DT5H6M7S"],
+    ["milliseconds", "P1Y2M3W4DT5H6M7.000S"],
+    ["microseconds", "P1Y2M3W4DT5H6M7.000000S"],
+    ["nanoseconds", "P1Y2M3W4DT5H6M7.000000000S"],
+  ],
+  "whole seconds toString"
+);
 
 const notValid = [
+  "era",
   "year",
   "month",
   "week",
@@ -24,5 +49,6 @@ const notValid = [
 ];
 
 notValid.forEach((smallestUnit) => {
-  assert.throws(RangeError, () => duration.toString({ smallestUnit }), smallestUnit);
+  assert.throws(RangeError, () => duration.toString({ smallestUnit }),
+    `"${smallestUnit}" is not a valid unit for the smallestUnit option`);
 });
diff --git a/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-invalid-string.js b/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-invalid-string.js
index f75db5a7ffc..0a1cc3e3cb8 100644
--- a/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-invalid-string.js
+++ b/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-invalid-string.js
@@ -10,10 +10,13 @@ info: |
     sec-temporal-tosecondsstringprecision step 9:
       9. Let _digits_ be ? GetStringOrNumberOption(_normalizedOptions_, *"fractionalSecondDigits"*, « *"auto"* », 0, 9, *"auto"*).
     sec-temporal.instant.prototype.tostring step 6:
-      6. Let _precision_ be ? ToDurationSecondsStringPrecision(_options_).
+      6. Let _precision_ be ? ToSecondsStringPrecision(_options_).
 features: [Temporal]
 ---*/
 
 const instant = new Temporal.Instant(1_000_000_000_987_650_000n);
 
-assert.throws(RangeError, () => instant.toString({ fractionalSecondDigits: "other string" }));
+for (const fractionalSecondDigits of ["other string", "AUTO", "not-auto", "autos", "auto\0"]) {
+  assert.throws(RangeError, () => instant.toString({ fractionalSecondDigits }),
+    `"${fractionalSecondDigits}" is not a valid value for fractionalSecondDigits`);
+}
diff --git a/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-non-integer.js b/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-non-integer.js
index beb567b6889..1a3479b5cc6 100644
--- a/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-non-integer.js
+++ b/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-non-integer.js
@@ -10,7 +10,7 @@ info: |
     sec-temporal-tosecondsstringprecision step 9:
       9. Let _digits_ be ? GetStringOrNumberOption(_normalizedOptions_, *"fractionalSecondDigits"*, « *"auto"* », 0, 9, *"auto"*).
     sec-temporal.instant.prototype.tostring step 6:
-      6. Let _precision_ be ? ToDurationSecondsStringPrecision(_options_).
+      6. Let _precision_ be ? ToSecondsStringPrecision(_options_).
 features: [Temporal]
 ---*/
 
diff --git a/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-out-of-range.js b/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-out-of-range.js
index 4506d3ef215..a634aee8903 100644
--- a/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-out-of-range.js
+++ b/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-out-of-range.js
@@ -10,13 +10,17 @@ info: |
     sec-temporal-tosecondsstringprecision step 9:
       9. Let _digits_ be ? GetStringOrNumberOption(_normalizedOptions_, *"fractionalSecondDigits"*, « *"auto"* », 0, 9, *"auto"*).
     sec-temporal.instant.prototype.tostring step 6:
-      6. Let _precision_ be ? ToDurationSecondsStringPrecision(_options_).
+      6. Let _precision_ be ? ToSecondsStringPrecision(_options_).
 features: [Temporal]
 ---*/
 
 const instant = new Temporal.Instant(1_000_000_000_987_650_000n);
 
-assert.throws(RangeError, () => instant.toString({ fractionalSecondDigits: -1 }));
-assert.throws(RangeError, () => instant.toString({ fractionalSecondDigits: 10 }));
-assert.throws(RangeError, () => instant.toString({ fractionalSecondDigits: -Infinity }));
-assert.throws(RangeError, () => instant.toString({ fractionalSecondDigits: Infinity }));
+assert.throws(RangeError, () => instant.toString({ fractionalSecondDigits: -Infinity }),
+  "−∞ is out of range for fractionalSecondDigits");
+assert.throws(RangeError, () => instant.toString({ fractionalSecondDigits: -1 }),
+  "−1 is out of range for fractionalSecondDigits");
+assert.throws(RangeError, () => instant.toString({ fractionalSecondDigits: 10 }),
+  "10 is out of range for fractionalSecondDigits");
+assert.throws(RangeError, () => instant.toString({ fractionalSecondDigits: Infinity }),
+  "∞ is out of range for fractionalSecondDigits");
diff --git a/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-undefined.js b/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-undefined.js
index 7bed467685f..be0fbf77e38 100644
--- a/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-undefined.js
+++ b/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-undefined.js
@@ -8,18 +8,31 @@ info: |
     sec-getoption step 3:
       3. If _value_ is *undefined*, return _fallback_.
     sec-getstringornumberoption step 2:
-      2. Let _value_ be ? GetOption(_options_, _property_, *"stringOrNumber"*, *undefined*, _fallback_).
+      2. Let _value_ be ? GetOption(_options_, _property_, « Number, String », *undefined*, _fallback_).
     sec-temporal-tosecondsstringprecision step 9:
       9. Let _digits_ be ? GetStringOrNumberOption(_normalizedOptions_, *"fractionalSecondDigits"*, « *"auto"* », 0, 9, *"auto"*).
     sec-temporal.instant.prototype.tostring step 6:
-      6. Let _precision_ be ? ToDurationSecondsStringPrecision(_options_).
+      6. Let _precision_ be ? ToSecondsStringPrecision(_options_).
 features: [Temporal]
 ---*/
 
-const instant = new Temporal.Instant(1_000_000_000_987_650_000n);
+const zeroSeconds = new Temporal.Instant(0n);
+const wholeSeconds = new Temporal.Instant(30_000_000_000n);
+const subSeconds = new Temporal.Instant(30_123_400_000n);
 
-const explicit = instant.toString({ fractionalSecondDigits: undefined });
-assert.sameValue(explicit, "2001-09-09T01:46:40.98765Z", "default fractionalSecondDigits is auto");
+const tests = [
+  [zeroSeconds, "1970-01-01T00:00:00Z"],
+  [wholeSeconds, "1970-01-01T00:00:30Z"],
+  [subSeconds, "1970-01-01T00:00:30.1234Z"],
+];
 
-const implicit = instant.toString({});
-assert.sameValue(implicit, "2001-09-09T01:46:40.98765Z", "default fractionalSecondDigits is auto");
+for (const [instant, expected] of tests) {
+  const explicit = instant.toString({ fractionalSecondDigits: undefined });
+  assert.sameValue(explicit, expected, "default fractionalSecondDigits is auto (property present but undefined)");
+
+  const implicit = instant.toString({});
+  assert.sameValue(implicit, expected, "default fractionalSecondDigits is auto (property not present)");
+
+  const lambda = instant.toString(() => {});
+  assert.sameValue(lambda, expected, "default fractionalSecondDigits is auto (property not present, function object)");
+}
diff --git a/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-wrong-type.js b/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-wrong-type.js
index 26eb23a3037..419b4a86d16 100644
--- a/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-wrong-type.js
+++ b/test/built-ins/Temporal/Instant/prototype/toString/fractionalseconddigits-wrong-type.js
@@ -22,4 +22,26 @@ features: [Temporal]
 ---*/
 
 const instant = new Temporal.Instant(1_000_000_000_987_650_000n);
-TemporalHelpers.checkFractionalSecondDigitsOptionWrongType(instant);
+
+assert.throws(RangeError, () => instant.toString({ fractionalSecondDigits: null }),
+  "null is not a number and converts to the string 'null' which is not valid for fractionalSecondDigits");
+assert.throws(RangeError, () => instant.toString({ fractionalSecondDigits: true }),
+  "true is not a number and converts to the string 'true' which is not valid for fractionalSecondDigits");
+assert.throws(RangeError, () => instant.toString({ fractionalSecondDigits: false }),
+  "false is not a number and converts to the string 'false' which is not valid for fractionalSecondDigits");
+assert.throws(TypeError, () => instant.toString({ fractionalSecondDigits: Symbol() }),
+  "symbols are not numbers and cannot convert to strings");
+assert.throws(RangeError, () => instant.toString({ fractionalSecondDigits: 2n }),
+  "bigints are not numbers and convert to strings which are not valid for fractionalSecondDigits");
+assert.throws(RangeError, () => instant.toString({ fractionalSecondDigits: {} }),
+  "plain objects are not numbers and convert to strings which are not valid for fractionalSecondDigits");
+
+const expected = [
+  "get fractionalSecondDigits.toString",
+  "call fractionalSecondDigits.toString",
+];
+const actual = [];
+const observer = TemporalHelpers.toPrimitiveObserver(actual, "auto", "fractionalSecondDigits");
+const result = instant.toString({ fractionalSecondDigits: observer });
+assert.sameValue(result, "2001-09-09T01:46:40.98765Z", "object with toString uses toString return value");
+assert.compareArray(actual, expected, "object with toString calls toString and not valueOf");
diff --git a/test/built-ins/Temporal/Instant/prototype/toString/smallestunit-valid-units.js b/test/built-ins/Temporal/Instant/prototype/toString/smallestunit-valid-units.js
index 191eec97ed4..e1c53c93eb6 100644
--- a/test/built-ins/Temporal/Instant/prototype/toString/smallestunit-valid-units.js
+++ b/test/built-ins/Temporal/Instant/prototype/toString/smallestunit-valid-units.js
@@ -9,11 +9,36 @@ features: [Temporal]
 
 const instant = new Temporal.Instant(1_000_000_000_123_456_789n);
 
-assert.sameValue(instant.toString({ smallestUnit: "minute" }), "2001-09-09T01:46Z");
-assert.sameValue(instant.toString({ smallestUnit: "second" }), "2001-09-09T01:46:40Z");
-assert.sameValue(instant.toString({ smallestUnit: "millisecond" }), "2001-09-09T01:46:40.123Z");
-assert.sameValue(instant.toString({ smallestUnit: "microsecond" }), "2001-09-09T01:46:40.123456Z");
-assert.sameValue(instant.toString({ smallestUnit: "nanosecond" }), "2001-09-09T01:46:40.123456789Z");
+function test(instance, expectations, description) {
+  for (const [smallestUnit, expectedResult] of expectations) {
+    assert.sameValue(instance.toString({ smallestUnit }), expectedResult,
+      `${description} with smallestUnit "${smallestUnit}"`);
+  }
+}
+
+test(
+  instant,
+  [
+    ["minute", "2001-09-09T01:46Z"],
+    ["second", "2001-09-09T01:46:40Z"],
+    ["millisecond", "2001-09-09T01:46:40.123Z"],
+    ["microsecond", "2001-09-09T01:46:40.123456Z"],
+    ["nanosecond", "2001-09-09T01:46:40.123456789Z"],
+  ],
+  "subseconds toString"
+);
+
+test(
+  new Temporal.Instant(999_999_960_000_000_000n),
+  [
+    ["minute", "2001-09-09T01:46Z"],
+    ["second", "2001-09-09T01:46:00Z"],
+    ["millisecond", "2001-09-09T01:46:00.000Z"],
+    ["microsecond", "2001-09-09T01:46:00.000000Z"],
+    ["nanosecond", "2001-09-09T01:46:00.000000000Z"],
+  ],
+  "whole minutes toString"
+);
 
 const notValid = [
   "era",
@@ -25,5 +50,6 @@ const notValid = [
 ];
 
 notValid.forEach((smallestUnit) => {
-  assert.throws(RangeError, () => instant.toString({ smallestUnit }), smallestUnit);
+  assert.throws(RangeError, () => instant.toString({ smallestUnit }),
+    `"${smallestUnit}" is not a valid unit for the smallestUnit option`);
 });
diff --git a/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-invalid-string.js b/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-invalid-string.js
index 150d868658a..9fa496c2955 100644
--- a/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-invalid-string.js
+++ b/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-invalid-string.js
@@ -10,10 +10,13 @@ info: |
     sec-temporal-tosecondsstringprecision step 9:
       9. Let _digits_ be ? GetStringOrNumberOption(_normalizedOptions_, *"fractionalSecondDigits"*, « *"auto"* », 0, 9, *"auto"*).
     sec-temporal.plaindatetime.prototype.tostring step 4:
-      4. Let _precision_ be ? ToDurationSecondsStringPrecision(_options_).
+      4. Let _precision_ be ? ToSecondsStringPrecision(_options_).
 features: [Temporal]
 ---*/
 
 const datetime = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 650, 0);
 
-assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: "other string" }));
+for (const fractionalSecondDigits of ["other string", "AUTO", "not-auto", "autos", "auto\0"]) {
+  assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits }),
+    `"${fractionalSecondDigits}" is not a valid value for fractionalSecondDigits`);
+}
diff --git a/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-non-integer.js b/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-non-integer.js
index 7de58c30b34..281d78f1abd 100644
--- a/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-non-integer.js
+++ b/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-non-integer.js
@@ -10,7 +10,7 @@ info: |
     sec-temporal-tosecondsstringprecision step 9:
       9. Let _digits_ be ? GetStringOrNumberOption(_normalizedOptions_, *"fractionalSecondDigits"*, « *"auto"* », 0, 9, *"auto"*).
     sec-temporal.plaindatetime.prototype.tostring step 4:
-      4. Let _precision_ be ? ToDurationSecondsStringPrecision(_options_).
+      4. Let _precision_ be ? ToSecondsStringPrecision(_options_).
 features: [Temporal]
 ---*/
 
diff --git a/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-out-of-range.js b/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-out-of-range.js
index 948d707eb35..2ccbafaeec0 100644
--- a/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-out-of-range.js
+++ b/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-out-of-range.js
@@ -10,11 +10,17 @@ info: |
     sec-temporal-tosecondsstringprecision step 9:
       9. Let _digits_ be ? GetStringOrNumberOption(_normalizedOptions_, *"fractionalSecondDigits"*, « *"auto"* », 0, 9, *"auto"*).
     sec-temporal.plaindatetime.prototype.tostring step 4:
-      4. Let _precision_ be ? ToDurationSecondsStringPrecision(_options_).
+      4. Let _precision_ be ? ToSecondsStringPrecision(_options_).
 features: [Temporal]
 ---*/
 
 const datetime = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 650, 0);
 
-assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: -1 }));
-assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: 10 }));
+assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: -Infinity }),
+  "−∞ is out of range for fractionalSecondDigits");
+assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: -1 }),
+  "−1 is out of range for fractionalSecondDigits");
+assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: 10 }),
+  "10 is out of range for fractionalSecondDigits");
+assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: Infinity }),
+  "∞ is out of range for fractionalSecondDigits");
diff --git a/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-undefined.js b/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-undefined.js
index 12a0f77b873..d5ebb9c7e68 100644
--- a/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-undefined.js
+++ b/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-undefined.js
@@ -8,17 +8,31 @@ info: |
     sec-getoption step 3:
       3. If _value_ is *undefined*, return _fallback_.
     sec-getstringornumberoption step 2:
-      2. Let _value_ be ? GetOption(_options_, _property_, *"stringOrNumber"*, *undefined*, _fallback_).
+      2. Let _value_ be ? GetOption(_options_, _property_, « Number, String », *undefined*, _fallback_).
     sec-temporal-tosecondsstringprecision step 9:
       9. Let _digits_ be ? GetStringOrNumberOption(_normalizedOptions_, *"fractionalSecondDigits"*, « *"auto"* », 0, 9, *"auto"*).
     sec-temporal.plaindatetime.prototype.tostring step 4:
-      4. Let _precision_ be ? ToDurationSecondsStringPrecision(_options_).
+      4. Let _precision_ be ? ToSecondsStringPrecision(_options_).
 features: [Temporal]
 ---*/
 
-const datetime = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 650, 0);
+const zeroSeconds = new Temporal.PlainDateTime(1976, 11, 18, 15, 23);
+const wholeSeconds = new Temporal.PlainDateTime(1976, 11, 18, 15, 23, 30);
+const subSeconds = new Temporal.PlainDateTime(1976, 11, 18, 15, 23, 30, 123, 400);
 
-const explicit = datetime.toString({ fractionalSecondDigits: undefined });
-assert.sameValue(explicit, "2000-05-02T12:34:56.98765", "default fractionalSecondDigits is auto");
+const tests = [
+  [zeroSeconds, "1976-11-18T15:23:00"],
+  [wholeSeconds, "1976-11-18T15:23:30"],
+  [subSeconds, "1976-11-18T15:23:30.1234"],
+];
 
-// See options-undefined.js for {}
+for (const [datetime, expected] of tests) {
+  const explicit = datetime.toString({ fractionalSecondDigits: undefined });
+  assert.sameValue(explicit, expected, "default fractionalSecondDigits is auto (property present but undefined)");
+
+  const implicit = datetime.toString({});
+  assert.sameValue(implicit, expected, "default fractionalSecondDigits is auto (property not present)");
+
+  const lambda = datetime.toString(() => {});
+  assert.sameValue(lambda, expected, "default fractionalSecondDigits is auto (property not present, function object)");
+}
diff --git a/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-wrong-type.js b/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-wrong-type.js
index 9177b82196a..5ecaf7a6d1f 100644
--- a/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-wrong-type.js
+++ b/test/built-ins/Temporal/PlainDateTime/prototype/toString/fractionalseconddigits-wrong-type.js
@@ -22,4 +22,26 @@ features: [Temporal]
 ---*/
 
 const datetime = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 987, 650, 0);
-TemporalHelpers.checkFractionalSecondDigitsOptionWrongType(datetime);
+
+assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: null }),
+  "null is not a number and converts to the string 'null' which is not valid for fractionalSecondDigits");
+assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: true }),
+  "true is not a number and converts to the string 'true' which is not valid for fractionalSecondDigits");
+assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: false }),
+  "false is not a number and converts to the string 'false' which is not valid for fractionalSecondDigits");
+assert.throws(TypeError, () => datetime.toString({ fractionalSecondDigits: Symbol() }),
+  "symbols are not numbers and cannot convert to strings");
+assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: 2n }),
+  "bigints are not numbers and convert to strings which are not valid for fractionalSecondDigits");
+assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: {} }),
+  "plain objects are not numbers and convert to strings which are not valid for fractionalSecondDigits");
+
+const expected = [
+  "get fractionalSecondDigits.toString",
+  "call fractionalSecondDigits.toString",
+];
+const actual = [];
+const observer = TemporalHelpers.toPrimitiveObserver(actual, "auto", "fractionalSecondDigits");
+const result = datetime.toString({ fractionalSecondDigits: observer });
+assert.sameValue(result, "2000-05-02T12:34:56.98765", "object with toString uses toString return value");
+assert.compareArray(actual, expected, "object with toString calls toString and not valueOf");
diff --git a/test/built-ins/Temporal/PlainDateTime/prototype/toString/smallestunit-valid-units.js b/test/built-ins/Temporal/PlainDateTime/prototype/toString/smallestunit-valid-units.js
index 34928a9c6d9..6aff7a613b2 100644
--- a/test/built-ins/Temporal/PlainDateTime/prototype/toString/smallestunit-valid-units.js
+++ b/test/built-ins/Temporal/PlainDateTime/prototype/toString/smallestunit-valid-units.js
@@ -7,15 +7,41 @@ description: Valid units for the smallestUnit option
 features: [Temporal]
 ---*/
 
-const datetime = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 789, 999, 999);
+const datetime = new Temporal.PlainDateTime(2000, 5, 2, 12, 34, 56, 123, 456, 789);
 
-assert.sameValue(datetime.toString({ smallestUnit: "minute" }), "2000-05-02T12:34");
-assert.sameValue(datetime.toString({ smallestUnit: "second" }), "2000-05-02T12:34:56");
-assert.sameValue(datetime.toString({ smallestUnit: "millisecond" }), "2000-05-02T12:34:56.789");
-assert.sameValue(datetime.toString({ smallestUnit: "microsecond" }), "2000-05-02T12:34:56.789999");
-assert.sameValue(datetime.toString({ smallestUnit: "nanosecond" }), "2000-05-02T12:34:56.789999999");
+function test(instance, expectations, description) {
+  for (const [smallestUnit, expectedResult] of expectations) {
+    assert.sameValue(instance.toString({ smallestUnit }), expectedResult,
+      `${description} with smallestUnit "${smallestUnit}"`);
+  }
+}
+
+test(
+  datetime,
+  [
+    ["minute", "2000-05-02T12:34"],
+    ["second", "2000-05-02T12:34:56"],
+    ["millisecond", "2000-05-02T12:34:56.123"],
+    ["microsecond", "2000-05-02T12:34:56.123456"],
+    ["nanosecond", "2000-05-02T12:34:56.123456789"],
+  ],
+  "subseconds toString"
+);
+
+test(
+  new Temporal.PlainDateTime(2000, 5, 2, 12, 34),
+  [
+    ["minute", "2000-05-02T12:34"],
+    ["second", "2000-05-02T12:34:00"],
+    ["millisecond", "2000-05-02T12:34:00.000"],
+    ["microsecond", "2000-05-02T12:34:00.000000"],
+    ["nanosecond", "2000-05-02T12:34:00.000000000"],
+  ],
+  "whole minutes toString"
+);
 
 const notValid = [
+  "era",
   "year",
   "month",
   "week",
@@ -24,5 +50,6 @@ const notValid = [
 ];
 
 notValid.forEach((smallestUnit) => {
-  assert.throws(RangeError, () => datetime.toString({ smallestUnit }), smallestUnit);
+  assert.throws(RangeError, () => datetime.toString({ smallestUnit }),
+    `"${smallestUnit}" is not a valid unit for the smallestUnit option`);
 });
diff --git a/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-invalid-string.js b/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-invalid-string.js
index 3d4ab17c430..1be21d324c7 100644
--- a/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-invalid-string.js
+++ b/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-invalid-string.js
@@ -10,12 +10,13 @@ info: |
     sec-temporal-tosecondsstringprecision step 9:
       9. Let _digits_ be ? GetStringOrNumberOption(_normalizedOptions_, *"fractionalSecondDigits"*, « *"auto"* », 0, 9, *"auto"*).
     sec-temporal.plaintime.prototype.tostring step 4:
-      4. Let _precision_ be ? ToDurationSecondsStringPrecision(_options_).
+      4. Let _precision_ be ? ToSecondsStringPrecision(_options_).
 features: [Temporal]
 ---*/
 
 const time = new Temporal.PlainTime(12, 34, 56, 987, 650, 0);
 
-for (const fractionalSecondDigits of ["other string", "AUTO", "not-auto", "autos"]) {
-  assert.throws(RangeError, () => time.toString({ fractionalSecondDigits }));
+for (const fractionalSecondDigits of ["other string", "AUTO", "not-auto", "autos", "auto\0"]) {
+  assert.throws(RangeError, () => time.toString({ fractionalSecondDigits }),
+    `"${fractionalSecondDigits}" is not a valid value for fractionalSecondDigits`);
 }
diff --git a/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-non-integer.js b/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-non-integer.js
index 9b7c5250e0d..3ca8fed5aa0 100644
--- a/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-non-integer.js
+++ b/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-non-integer.js
@@ -10,7 +10,7 @@ info: |
     sec-temporal-tosecondsstringprecision step 9:
       9. Let _digits_ be ? GetStringOrNumberOption(_normalizedOptions_, *"fractionalSecondDigits"*, « *"auto"* », 0, 9, *"auto"*).
     sec-temporal.plaintime.prototype.tostring step 4:
-      4. Let _precision_ be ? ToDurationSecondsStringPrecision(_options_).
+      4. Let _precision_ be ? ToSecondsStringPrecision(_options_).
 features: [Temporal]
 ---*/
 
diff --git a/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-out-of-range.js b/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-out-of-range.js
index 5959dd94591..1d6e899312f 100644
--- a/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-out-of-range.js
+++ b/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-out-of-range.js
@@ -10,13 +10,17 @@ info: |
     sec-temporal-tosecondsstringprecision step 9:
       9. Let _digits_ be ? GetStringOrNumberOption(_normalizedOptions_, *"fractionalSecondDigits"*, « *"auto"* », 0, 9, *"auto"*).
     sec-temporal.plaintime.prototype.tostring step 4:
-      4. Let _precision_ be ? ToDurationSecondsStringPrecision(_options_).
+      4. Let _precision_ be ? ToSecondsStringPrecision(_options_).
 features: [Temporal]
 ---*/
 
 const time = new Temporal.PlainTime(12, 34, 56, 987, 650, 0);
 
-assert.throws(RangeError, () => time.toString({ fractionalSecondDigits: -Infinity }));
-assert.throws(RangeError, () => time.toString({ fractionalSecondDigits: -1 }));
-assert.throws(RangeError, () => time.toString({ fractionalSecondDigits: 10 }));
-assert.throws(RangeError, () => time.toString({ fractionalSecondDigits: Infinity }));
+assert.throws(RangeError, () => time.toString({ fractionalSecondDigits: -Infinity }),
+  "−∞ is out of range for fractionalSecondDigits");
+assert.throws(RangeError, () => time.toString({ fractionalSecondDigits: -1 }),
+  "−1 is out of range for fractionalSecondDigits");
+assert.throws(RangeError, () => time.toString({ fractionalSecondDigits: 10 }),
+  "10 is out of range for fractionalSecondDigits");
+assert.throws(RangeError, () => time.toString({ fractionalSecondDigits: Infinity }),
+  "∞ is out of range for fractionalSecondDigits");
diff --git a/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-undefined.js b/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-undefined.js
index 1d9d2dff038..6c33c55f7f4 100644
--- a/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-undefined.js
+++ b/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-undefined.js
@@ -8,29 +8,31 @@ info: |
     sec-getoption step 3:
       3. If _value_ is *undefined*, return _fallback_.
     sec-getstringornumberoption step 2:
-      2. Let _value_ be ? GetOption(_options_, _property_, *"stringOrNumber"*, *undefined*, _fallback_).
+      2. Let _value_ be ? GetOption(_options_, _property_, « Number, String », *undefined*, _fallback_).
     sec-temporal-tosecondsstringprecision step 9:
       9. Let _digits_ be ? GetStringOrNumberOption(_normalizedOptions_, *"fractionalSecondDigits"*, « *"auto"* », 0, 9, *"auto"*).
     sec-temporal.plaintime.prototype.tostring step 4:
-      4. Let _precision_ be ? ToDurationSecondsStringPrecision(_options_).
+      4. Let _precision_ be ? ToSecondsStringPrecision(_options_).
 features: [Temporal]
 ---*/
 
+const zeroSeconds = new Temporal.PlainTime(15, 23);
+const wholeSeconds = new Temporal.PlainTime(15, 23, 30);
+const subSeconds = new Temporal.PlainTime(15, 23, 30, 123, 400);
+
 const tests = [
-  ["15:23", "15:23:00"],
-  ["15:23:30", "15:23:30"],
-  ["15:23:30.1234", "15:23:30.1234"],
+  [zeroSeconds, "15:23:00"],
+  [wholeSeconds, "15:23:30"],
+  [subSeconds, "15:23:30.1234"],
 ];
 
-for (const [input, expected] of tests) {
-  const time = Temporal.PlainTime.from(input);
-
+for (const [time, expected] of tests) {
   const explicit = time.toString({ fractionalSecondDigits: undefined });
-  assert.sameValue(explicit, expected, "default fractionalSecondDigits is auto");
+  assert.sameValue(explicit, expected, "default fractionalSecondDigits is auto (property present but undefined)");
 
   const implicit = time.toString({});
-  assert.sameValue(implicit, expected, "default fractionalSecondDigits is auto");
+  assert.sameValue(implicit, expected, "default fractionalSecondDigits is auto (property not present)");
 
   const lambda = time.toString(() => {});
-  assert.sameValue(lambda, expected, "default fractionalSecondDigits is auto");
+  assert.sameValue(lambda, expected, "default fractionalSecondDigits is auto (property not present, function object)");
 }
diff --git a/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-wrong-type.js b/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-wrong-type.js
index 54440155aa3..79fcd282938 100644
--- a/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-wrong-type.js
+++ b/test/built-ins/Temporal/PlainTime/prototype/toString/fractionalseconddigits-wrong-type.js
@@ -22,4 +22,26 @@ features: [Temporal]
 ---*/
 
 const time = new Temporal.PlainTime(12, 34, 56, 987, 650, 0);
-TemporalHelpers.checkFractionalSecondDigitsOptionWrongType(time);
+
+assert.throws(RangeError, () => time.toString({ fractionalSecondDigits: null }),
+  "null is not a number and converts to the string 'null' which is not valid for fractionalSecondDigits");
+assert.throws(RangeError, () => time.toString({ fractionalSecondDigits: true }),
+  "true is not a number and converts to the string 'true' which is not valid for fractionalSecondDigits");
+assert.throws(RangeError, () => time.toString({ fractionalSecondDigits: false }),
+  "false is not a number and converts to the string 'false' which is not valid for fractionalSecondDigits");
+assert.throws(TypeError, () => time.toString({ fractionalSecondDigits: Symbol() }),
+  "symbols are not numbers and cannot convert to strings");
+assert.throws(RangeError, () => time.toString({ fractionalSecondDigits: 2n }),
+  "bigints are not numbers and convert to strings which are not valid for fractionalSecondDigits");
+assert.throws(RangeError, () => time.toString({ fractionalSecondDigits: {} }),
+  "plain objects are not numbers and convert to strings which are not valid for fractionalSecondDigits");
+
+const expected = [
+  "get fractionalSecondDigits.toString",
+  "call fractionalSecondDigits.toString",
+];
+const actual = [];
+const observer = TemporalHelpers.toPrimitiveObserver(actual, "auto", "fractionalSecondDigits");
+const result = time.toString({ fractionalSecondDigits: observer });
+assert.sameValue(result, "12:34:56.98765", "object with toString uses toString return value");
+assert.compareArray(actual, expected, "object with toString calls toString and not valueOf");
diff --git a/test/built-ins/Temporal/PlainTime/prototype/toString/smallestunit-valid-units.js b/test/built-ins/Temporal/PlainTime/prototype/toString/smallestunit-valid-units.js
index e2b4c71dbd2..bb942179e2c 100644
--- a/test/built-ins/Temporal/PlainTime/prototype/toString/smallestunit-valid-units.js
+++ b/test/built-ins/Temporal/PlainTime/prototype/toString/smallestunit-valid-units.js
@@ -7,21 +7,41 @@ description: Valid units for the smallestUnit option
 features: [Temporal]
 ---*/
 
-const time = new Temporal.PlainTime(12, 34, 56, 789, 999, 999);
-assert.sameValue(time.toString({ smallestUnit: "minute" }), "12:34");
-assert.sameValue(time.toString({ smallestUnit: "second" }), "12:34:56");
-assert.sameValue(time.toString({ smallestUnit: "millisecond" }), "12:34:56.789");
-assert.sameValue(time.toString({ smallestUnit: "microsecond" }), "12:34:56.789999");
-assert.sameValue(time.toString({ smallestUnit: "nanosecond" }), "12:34:56.789999999");
+const time = new Temporal.PlainTime(12, 34, 56, 123, 456, 789);
 
-const time2 = new Temporal.PlainTime(12, 34);
-assert.sameValue(time2.toString({ smallestUnit: "minute" }), "12:34");
-assert.sameValue(time2.toString({ smallestUnit: "second" }), "12:34:00");
-assert.sameValue(time2.toString({ smallestUnit: "millisecond" }), "12:34:00.000");
-assert.sameValue(time2.toString({ smallestUnit: "microsecond" }), "12:34:00.000000");
-assert.sameValue(time2.toString({ smallestUnit: "nanosecond" }), "12:34:00.000000000");
+function test(instance, expectations, description) {
+  for (const [smallestUnit, expectedResult] of expectations) {
+    assert.sameValue(instance.toString({ smallestUnit }), expectedResult,
+      `${description} with smallestUnit "${smallestUnit}"`);
+  }
+}
+
+test(
+  time,
+  [
+    ["minute", "12:34"],
+    ["second", "12:34:56"],
+    ["millisecond", "12:34:56.123"],
+    ["microsecond", "12:34:56.123456"],
+    ["nanosecond", "12:34:56.123456789"],
+  ],
+  "subseconds toString"
+);
+
+test(
+  new Temporal.PlainTime(12, 34),
+  [
+    ["minute", "12:34"],
+    ["second", "12:34:00"],
+    ["millisecond", "12:34:00.000"],
+    ["microsecond", "12:34:00.000000"],
+    ["nanosecond", "12:34:00.000000000"],
+  ],
+  "whole minutes toString"
+);
 
 const notValid = [
+  "era",
   "year",
   "month",
   "week",
@@ -30,5 +50,6 @@ const notValid = [
 ];
 
 notValid.forEach((smallestUnit) => {
-  assert.throws(RangeError, () => time.toString({ smallestUnit }), smallestUnit);
+  assert.throws(RangeError, () => time.toString({ smallestUnit }),
+    `"${smallestUnit}" is not a valid unit for the smallestUnit option`);
 });
diff --git a/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-invalid-string.js b/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-invalid-string.js
index ea172ceb80e..c933354b9d4 100644
--- a/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-invalid-string.js
+++ b/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-invalid-string.js
@@ -9,11 +9,14 @@ info: |
       4. If _stringValues_ is not *undefined* and _stringValues_ does not contain an element equal to _value_, throw a *RangeError* exception.
     sec-temporal-tosecondsstringprecision step 9:
       9. Let _digits_ be ? GetStringOrNumberOption(_normalizedOptions_, *"fractionalSecondDigits"*, « *"auto"* », 0, 9, *"auto"*).
-    sec-temporal.instant.prototype.tostring step 4:
-      4. Let _precision_ be ? ToDurationSecondsStringPrecision(_options_).
+    sec-temporal.zoneddatetime.prototype.tostring step 4:
+      4. Let _precision_ be ? ToSecondsStringPrecision(_options_).
 features: [Temporal]
 ---*/
 
 const datetime = new Temporal.ZonedDateTime(1_000_000_000_987_650_000n, "UTC");
 
-assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: "other string" }));
+for (const fractionalSecondDigits of ["other string", "AUTO", "not-auto", "autos", "auto\0"]) {
+  assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits }),
+    `"${fractionalSecondDigits}" is not a valid value for fractionalSecondDigits`);
+}
diff --git a/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-non-integer.js b/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-non-integer.js
index 93685aedece..a9ccce61d8d 100644
--- a/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-non-integer.js
+++ b/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-non-integer.js
@@ -10,7 +10,7 @@ info: |
     sec-temporal-tosecondsstringprecision step 9:
       9. Let _digits_ be ? GetStringOrNumberOption(_normalizedOptions_, *"fractionalSecondDigits"*, « *"auto"* », 0, 9, *"auto"*).
     sec-temporal.zoneddatetime.prototype.tostring step 4:
-      4. Let _precision_ be ? ToDurationSecondsStringPrecision(_options_).
+      4. Let _precision_ be ? ToSecondsStringPrecision(_options_).
 features: [Temporal]
 ---*/
 
diff --git a/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-out-of-range.js b/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-out-of-range.js
index 36ae8d14293..487600a4158 100644
--- a/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-out-of-range.js
+++ b/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-out-of-range.js
@@ -10,11 +10,17 @@ info: |
     sec-temporal-tosecondsstringprecision step 9:
       9. Let _digits_ be ? GetStringOrNumberOption(_normalizedOptions_, *"fractionalSecondDigits"*, « *"auto"* », 0, 9, *"auto"*).
     sec-temporal.zoneddatetime.prototype.tostring step 4:
-      4. Let _precision_ be ? ToDurationSecondsStringPrecision(_options_).
+      4. Let _precision_ be ? ToSecondsStringPrecision(_options_).
 features: [Temporal]
 ---*/
 
 const datetime = new Temporal.ZonedDateTime(1_000_000_000_987_650_000n, "UTC");
 
-assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: -1 }));
-assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: 10 }));
+assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: -Infinity }),
+  "−∞ is out of range for fractionalSecondDigits");
+assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: -1 }),
+  "−1 is out of range for fractionalSecondDigits");
+assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: 10 }),
+  "10 is out of range for fractionalSecondDigits");
+assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: Infinity }),
+  "∞ is out of range for fractionalSecondDigits");
diff --git a/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-undefined.js b/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-undefined.js
index a4e0a10b5f0..734f436c4c3 100644
--- a/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-undefined.js
+++ b/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-undefined.js
@@ -8,17 +8,31 @@ info: |
     sec-getoption step 3:
       3. If _value_ is *undefined*, return _fallback_.
     sec-getstringornumberoption step 2:
-      2. Let _value_ be ? GetOption(_options_, _property_, *"stringOrNumber"*, *undefined*, _fallback_).
+      2. Let _value_ be ? GetOption(_options_, _property_, « Number, String », *undefined*, _fallback_).
     sec-temporal-tosecondsstringprecision step 9:
       9. Let _digits_ be ? GetStringOrNumberOption(_normalizedOptions_, *"fractionalSecondDigits"*, « *"auto"* », 0, 9, *"auto"*).
     sec-temporal.zoneddatetime.prototype.tostring step 4:
-      4. Let _precision_ be ? ToDurationSecondsStringPrecision(_options_).
+      4. Let _precision_ be ? ToSecondsStringPrecision(_options_).
 features: [Temporal]
 ---*/
 
-const datetime = new Temporal.ZonedDateTime(1_000_000_000_987_650_000n, "UTC");
+const zeroSeconds = new Temporal.ZonedDateTime(0n, "UTC");
+const wholeSeconds = new Temporal.ZonedDateTime(30_000_000_000n, "UTC");
+const subSeconds = new Temporal.ZonedDateTime(30_123_400_000n, "UTC");
 
-const explicit = datetime.toString({ fractionalSecondDigits: undefined });
-assert.sameValue(explicit, "2001-09-09T01:46:40.98765+00:00[UTC]", "default fractionalSecondDigits is auto");
+const tests = [
+  [zeroSeconds, "1970-01-01T00:00:00+00:00[UTC]"],
+  [wholeSeconds, "1970-01-01T00:00:30+00:00[UTC]"],
+  [subSeconds, "1970-01-01T00:00:30.1234+00:00[UTC]"],
+];
 
-// See options-undefined.js for {}
+for (const [datetime, expected] of tests) {
+  const explicit = datetime.toString({ fractionalSecondDigits: undefined });
+  assert.sameValue(explicit, expected, "default fractionalSecondDigits is auto (property present but undefined)");
+
+  const implicit = datetime.toString({});
+  assert.sameValue(implicit, expected, "default fractionalSecondDigits is auto (property not present)");
+
+  const lambda = datetime.toString(() => {});
+  assert.sameValue(lambda, expected, "default fractionalSecondDigits is auto (property not present, function object)");
+}
diff --git a/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-wrong-type.js b/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-wrong-type.js
index b147dbac3e2..9a12691ad6d 100644
--- a/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-wrong-type.js
+++ b/test/built-ins/Temporal/ZonedDateTime/prototype/toString/fractionalseconddigits-wrong-type.js
@@ -22,4 +22,26 @@ features: [Temporal]
 ---*/
 
 const datetime = new Temporal.ZonedDateTime(1_000_000_000_987_650_000n, "UTC");
-TemporalHelpers.checkFractionalSecondDigitsOptionWrongType(datetime);
+
+assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: null }),
+  "null is not a number and converts to the string 'null' which is not valid for fractionalSecondDigits");
+assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: true }),
+  "true is not a number and converts to the string 'true' which is not valid for fractionalSecondDigits");
+assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: false }),
+  "false is not a number and converts to the string 'false' which is not valid for fractionalSecondDigits");
+assert.throws(TypeError, () => datetime.toString({ fractionalSecondDigits: Symbol() }),
+  "symbols are not numbers and cannot convert to strings");
+assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: 2n }),
+  "bigints are not numbers and convert to strings which are not valid for fractionalSecondDigits");
+assert.throws(RangeError, () => datetime.toString({ fractionalSecondDigits: {} }),
+  "plain objects are not numbers and convert to strings which are not valid for fractionalSecondDigits");
+
+const expected = [
+  "get fractionalSecondDigits.toString",
+  "call fractionalSecondDigits.toString",
+];
+const actual = [];
+const observer = TemporalHelpers.toPrimitiveObserver(actual, "auto", "fractionalSecondDigits");
+const result = datetime.toString({ fractionalSecondDigits: observer });
+assert.sameValue(result, "2001-09-09T01:46:40.98765+00:00[UTC]", "object with toString uses toString return value");
+assert.compareArray(actual, expected, "object with toString calls toString and not valueOf");
diff --git a/test/built-ins/Temporal/ZonedDateTime/prototype/toString/smallestunit-valid-units.js b/test/built-ins/Temporal/ZonedDateTime/prototype/toString/smallestunit-valid-units.js
index 12c10368b64..6ea787955a9 100644
--- a/test/built-ins/Temporal/ZonedDateTime/prototype/toString/smallestunit-valid-units.js
+++ b/test/built-ins/Temporal/ZonedDateTime/prototype/toString/smallestunit-valid-units.js
@@ -9,13 +9,39 @@ features: [Temporal]
 
 const datetime = new Temporal.ZonedDateTime(1_000_000_000_123_456_789n, "UTC");
 
-assert.sameValue(datetime.toString({ smallestUnit: "minute" }), "2001-09-09T01:46+00:00[UTC]");
-assert.sameValue(datetime.toString({ smallestUnit: "second" }), "2001-09-09T01:46:40+00:00[UTC]");
-assert.sameValue(datetime.toString({ smallestUnit: "millisecond" }), "2001-09-09T01:46:40.123+00:00[UTC]");
-assert.sameValue(datetime.toString({ smallestUnit: "microsecond" }), "2001-09-09T01:46:40.123456+00:00[UTC]");
-assert.sameValue(datetime.toString({ smallestUnit: "nanosecond" }), "2001-09-09T01:46:40.123456789+00:00[UTC]");
+function test(instance, expectations, description) {
+  for (const [smallestUnit, expectedResult] of expectations) {
+    assert.sameValue(instance.toString({ smallestUnit }), expectedResult,
+      `${description} with smallestUnit "${smallestUnit}"`);
+  }
+}
+
+test(
+  datetime,
+  [
+    ["minute", "2001-09-09T01:46+00:00[UTC]"],
+    ["second", "2001-09-09T01:46:40+00:00[UTC]"],
+    ["millisecond", "2001-09-09T01:46:40.123+00:00[UTC]"],
+    ["microsecond", "2001-09-09T01:46:40.123456+00:00[UTC]"],
+    ["nanosecond", "2001-09-09T01:46:40.123456789+00:00[UTC]"],
+  ],
+  "subseconds toString"
+);
+
+test(
+  new Temporal.ZonedDateTime(999_999_960_000_000_000n, "UTC"),
+  [
+    ["minute", "2001-09-09T01:46+00:00[UTC]"],
+    ["second", "2001-09-09T01:46:00+00:00[UTC]"],
+    ["millisecond", "2001-09-09T01:46:00.000+00:00[UTC]"],
+    ["microsecond", "2001-09-09T01:46:00.000000+00:00[UTC]"],
+    ["nanosecond", "2001-09-09T01:46:00.000000000+00:00[UTC]"],
+  ],
+  "whole minutes toString"
+);
 
 const notValid = [
+  "era",
   "year",
   "month",
   "week",
@@ -24,5 +50,6 @@ const notValid = [
 ];
 
 notValid.forEach((smallestUnit) => {
-  assert.throws(RangeError, () => datetime.toString({ smallestUnit }), smallestUnit);
+  assert.throws(RangeError, () => datetime.toString({ smallestUnit }),
+    `"${smallestUnit}" is not a valid unit for the smallestUnit option`);
 });