diff --git a/test/built-ins/Temporal/Calendar/prototype/monthsInYear/date-time.js b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/date-time.js
new file mode 100644
index 00000000000..e45fd322426
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/date-time.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: Temporal.Calendar.prototype.monthsInYear will take Temporal.PlainDateTime
+  object and return the number 12 for ISO8601 calendar.
+info: |
+  4. If Type(temporalDateLike) is not Object or temporalDateLike does not have an [[InitializedTemporalDate]] or [[InitializedTemporalYearMonth]] internal slot, then
+    a. Perform ? ToTemporalDate(temporalDateLike).
+  5. Return 12𝔽.
+features: [Temporal]
+---*/
+let cal = new Temporal.Calendar("iso8601");
+
+let dt = new Temporal.PlainDateTime(1997, 8, 23, 5, 30, 13);
+assert.sameValue(cal.monthsInYear(dt), 12);
+dt = new Temporal.PlainDateTime(1221, 8, 23, 5, 30, 13);
+assert.sameValue(cal.monthsInYear(dt), 12);
diff --git a/test/built-ins/Temporal/Calendar/prototype/monthsInYear/date.js b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/date.js
new file mode 100644
index 00000000000..af4a0988fec
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/date.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: Temporal.Calendar.prototype.monthsInYear will take Temporal.PlainDate
+  and return the number 12 for ISO8601 calendar.
+info: |
+  4. If Type(temporalDateLike) is not Object or temporalDateLike does not have an [[InitializedTemporalDate]] or [[InitializedTemporalYearMonth]] internal slot, then
+    a. Perform ? ToTemporalDate(temporalDateLike).
+  5. Return 12𝔽.
+features: [Temporal]
+---*/
+let cal = new Temporal.Calendar("iso8601");
+
+let d = new Temporal.PlainDate(2021, 7, 15);
+assert.sameValue(12, cal.monthsInYear(d), 12);
+d = new Temporal.PlainDate(1234, 12, 3);
+assert.sameValue(12, cal.monthsInYear(d), 12);
diff --git a/test/built-ins/Temporal/Calendar/prototype/monthsInYear/string-throw-range-error.js b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/string-throw-range-error.js
new file mode 100644
index 00000000000..8aaa2702a8e
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/string-throw-range-error.js
@@ -0,0 +1,17 @@
+// 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: Temporal.Calendar.prototype.monthsInYear will throw RangeError with incorrect ISO8601 string.
+info: |
+  4. If Type(temporalDateLike) is not Object or temporalDateLike does not have an [[InitializedTemporalDate]] or [[InitializedTemporalYearMonth]] internal slot, then
+  a. Perform ? ToTemporalDate(temporalDateLike).
+  5. Return 12𝔽.
+features: [Temporal]
+---*/
+let cal = new Temporal.Calendar("iso8601");
+
+assert.throws(RangeError, () => cal.monthsInYear("2021-01"), "Missing day");
+assert.throws(RangeError, () => cal.monthsInYear("2019-12"), "Missing day");
+assert.throws(RangeError, () => cal.monthsInYear("P1Y"), "Duration is not acceptable");
+assert.throws(RangeError, () => cal.monthsInYear("-P12Y"), "Duration is not acceptable");
diff --git a/test/built-ins/Temporal/Calendar/prototype/monthsInYear/string.js b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/string.js
new file mode 100644
index 00000000000..e4d5e9570e5
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/string.js
@@ -0,0 +1,17 @@
+// 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: Temporal.Calendar.prototype.monthsInYear will take ISO8601 string
+  and return the number 12 for ISO8601 calendar.
+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("3456-12-20T03:04:05Z"), 12);
+assert.sameValue(cal.monthsInYear("+000998-01-28T03:04:05Z"), 12);
diff --git a/test/built-ins/Temporal/Calendar/prototype/monthsInYear/throw-range-error-ToTemporalDate.js b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/throw-range-error-ToTemporalDate.js
new file mode 100644
index 00000000000..cdad67fc7bc
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/throw-range-error-ToTemporalDate.js
@@ -0,0 +1,17 @@
+// 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: Temporal.Calendar.prototype.monthsInYear throws RangeError on
+  ToTemporalDate when temporalDateLike is invalid string.
+info: |
+  4. If Type(temporalDateLike) is not Object or temporalDateLike
+     does not have an [[InitializedTemporalDate]] or
+     [[InitializedTemporalYearMonth]] internal slot, then
+    a. Set temporalDateLike to ? ToTemporalDate(temporalDateLike).
+features: [Temporal]
+---*/
+let cal = new Temporal.Calendar("iso8601");
+
+assert.throws(RangeError, () => cal.monthsInYear("invalid string"),
+    "Throw RangeError if temporalDateLike is invalid");
diff --git a/test/built-ins/Temporal/Calendar/prototype/monthsInYear/throw-type-error-RequireInternalSlot.js b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/throw-type-error-RequireInternalSlot.js
new file mode 100644
index 00000000000..ac3ce1e5f92
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/throw-type-error-RequireInternalSlot.js
@@ -0,0 +1,15 @@
+// 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: Temporal.Calendar.prototype.monthsInYear throws TypeError on
+  RequireInternalSlot if object has no internal slot.
+info: |
+  2. Perform ? RequireInternalSlot(calendar, [[InitializedTemporalCalendar]]).
+features: [Temporal]
+---*/
+let cal = new Temporal.Calendar("iso8601");
+
+let badCal = { monthsInYear: cal.monthsInYear }
+assert.throws(TypeError, () => badCal.monthsInYear("2021-03-04"),
+    "Throw TypeError if there are no internal slot");
diff --git a/test/built-ins/Temporal/Calendar/prototype/monthsInYear/year-month.js b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/year-month.js
new file mode 100644
index 00000000000..884e5e4b811
--- /dev/null
+++ b/test/built-ins/Temporal/Calendar/prototype/monthsInYear/year-month.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: Temporal.Calendar.prototype.monthsInYear will take
+  PlainYearMonth object and return the number 12 for ISO8601 calendar.
+info: |
+  4. If Type(temporalDateLike) is not Object or temporalDateLike does not have an [[InitializedTemporalDate]] or [[InitializedTemporalYearMonth]] internal slot, then
+  a. Perform ? ToTemporalDate(temporalDateLike).
+  5. Return 12𝔽.
+features: [Temporal]
+---*/
+let cal = new Temporal.Calendar("iso8601");
+
+let ym = new Temporal.PlainYearMonth(1, 1);
+assert.sameValue(cal.monthsInYear(ym), 12);
+ym = new Temporal.PlainYearMonth(9384, 3);
+assert.sameValue(cal.monthsInYear(ym), 12);