diff --git a/test/built-ins/Temporal/Calendar/prototype/monthsInYear/argument-string.js b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/argument-string.js
new file mode 100644
index 00000000000..9a8748d8771
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/argument-string.js
@@ -0,0 +1,18 @@
+// Copyright (C) 2021 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.monthsinyear
+description: An ISO 8601 date string should be converted as input
+info: |
+  a. Perform ? ToTemporalDate(temporalDateLike).
+  5. Return 12𝔽.
+features: [Temporal]
+---*/
+
+let cal = new Temporal.Calendar("iso8601");
+
+assert.sameValue(cal.monthsInYear("3456-12-20"), 12);
+assert.sameValue(cal.monthsInYear("+000998-01-28"), 12);
+assert.sameValue(cal.monthsInYear("3456-12-20T03:04:05+00:00[UTC]"), 12);
+assert.sameValue(cal.monthsInYear("+000998-01-28T03:04:05+00:00[UTC]"), 12);
diff --git a/test/built-ins/Temporal/Calendar/prototype/monthsInYear/basic.js b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/basic.js
index fcf4e35084a..9edcee17457 100644
--- a/test/built-ins/Temporal/Calendar/prototype/monthsInYear/basic.js
+++ b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/basic.js
@@ -9,9 +9,9 @@ features: [Temporal]
 
 const iso = Temporal.Calendar.from("iso8601");
 const res = 12;
-assert.sameValue(iso.monthsInYear(Temporal.PlainDate.from("1994-11-05")), res, "PlainDate");
-assert.sameValue(iso.monthsInYear(Temporal.PlainDateTime.from("1994-11-05T08:15:30")), res, "PlainDateTime");
-assert.sameValue(iso.monthsInYear(Temporal.PlainYearMonth.from("1994-11")), res, "PlainYearMonth");
+assert.sameValue(iso.monthsInYear(new Temporal.PlainDate(1994, 11, 5)), res, "PlainDate");
+assert.sameValue(iso.monthsInYear(new Temporal.PlainDateTime(1994, 11, 5, 8, 15, 30)), res, "PlainDateTime");
+assert.sameValue(iso.monthsInYear(new Temporal.PlainYearMonth(1994, 11)), res, "PlainYearMonth");
 assert.sameValue(iso.monthsInYear({ year: 1994, month: 11, day: 5 }), res, "property bag");
 assert.sameValue(iso.monthsInYear("1994-11-05"), res, "string");
 assert.throws(TypeError, () => iso.monthsInYear({ year: 2000 }), "property bag with missing properties");
diff --git a/test/built-ins/Temporal/Calendar/prototype/monthsInYear/branding.js b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/branding.js
index e86c5f50e22..4d16a000786 100644
--- a/test/built-ins/Temporal/Calendar/prototype/monthsInYear/branding.js
+++ b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/branding.js
@@ -11,12 +11,14 @@ const monthsInYear = Temporal.Calendar.prototype.monthsInYear;
 
 assert.sameValue(typeof monthsInYear, "function");
 
-assert.throws(TypeError, () => monthsInYear.call(undefined), "undefined");
-assert.throws(TypeError, () => monthsInYear.call(null), "null");
-assert.throws(TypeError, () => monthsInYear.call(true), "true");
-assert.throws(TypeError, () => monthsInYear.call(""), "empty string");
-assert.throws(TypeError, () => monthsInYear.call(Symbol()), "symbol");
-assert.throws(TypeError, () => monthsInYear.call(1), "1");
-assert.throws(TypeError, () => monthsInYear.call({}), "plain object");
-assert.throws(TypeError, () => monthsInYear.call(Temporal.Calendar), "Temporal.Calendar");
-assert.throws(TypeError, () => monthsInYear.call(Temporal.Calendar.prototype), "Temporal.Calendar.prototype");
+const arg = new Temporal.PlainDate(2021, 3, 4);
+
+assert.throws(TypeError, () => monthsInYear.call(undefined, arg), "undefined");
+assert.throws(TypeError, () => monthsInYear.call(null, arg), "null");
+assert.throws(TypeError, () => monthsInYear.call(true, arg), "true");
+assert.throws(TypeError, () => monthsInYear.call("", arg), "empty string");
+assert.throws(TypeError, () => monthsInYear.call(Symbol(), arg), "symbol");
+assert.throws(TypeError, () => monthsInYear.call(1, arg), "1");
+assert.throws(TypeError, () => monthsInYear.call({}, arg), "plain object");
+assert.throws(TypeError, () => monthsInYear.call(Temporal.Calendar, arg), "Temporal.Calendar");
+assert.throws(TypeError, () => monthsInYear.call(Temporal.Calendar.prototype, arg), "Temporal.Calendar.prototype");