diff --git a/Libraries/LibJS/Runtime/Temporal/PlainMonthDayPrototype.cpp b/Libraries/LibJS/Runtime/Temporal/PlainMonthDayPrototype.cpp
index 325dc5e7745..97d4b327e1e 100644
--- a/Libraries/LibJS/Runtime/Temporal/PlainMonthDayPrototype.cpp
+++ b/Libraries/LibJS/Runtime/Temporal/PlainMonthDayPrototype.cpp
@@ -41,6 +41,7 @@ void PlainMonthDayPrototype::initialize(Realm& realm)
     define_native_function(realm, vm.names.toLocaleString, to_locale_string, 0, attr);
     define_native_function(realm, vm.names.toJSON, to_json, 0, attr);
     define_native_function(realm, vm.names.valueOf, value_of, 0, attr);
+    define_native_function(realm, vm.names.toPlainDate, to_plain_date, 1, attr);
 }
 
 // 10.3.3 get Temporal.PlainMonthDay.prototype.calendarId, https://tc39.es/proposal-temporal/#sec-get-temporal.plainmonthday.prototype.calendarid
@@ -180,4 +181,38 @@ JS_DEFINE_NATIVE_FUNCTION(PlainMonthDayPrototype::value_of)
     return vm.throw_completion<TypeError>(ErrorType::Convert, "Temporal.PlainMonthDay", "a primitive value");
 }
 
+// 10.3.12 Temporal.PlainMonthDay.prototype.toPlainDate ( item ), https://tc39.es/proposal-temporal/#sec-temporal.plainmonthday.prototype.toplaindate
+JS_DEFINE_NATIVE_FUNCTION(PlainMonthDayPrototype::to_plain_date)
+{
+    auto item = vm.argument(0);
+
+    // 1. Let monthDay be the this value.
+    // 2. Perform ? RequireInternalSlot(monthDay, [[InitializedTemporalMonthDay]]).
+    auto month_day = TRY(typed_this_object(vm));
+
+    // 3. If item is not an Object, then
+    if (!item.is_object()) {
+        // a. Throw a TypeError exception.
+        return vm.throw_completion<TypeError>(ErrorType::NotAnObject, item);
+    }
+
+    // 4. Let calendar be monthDay.[[Calendar]].
+    auto const& calendar = month_day->calendar();
+
+    // 5. Let fields be ISODateToFields(calendar, monthDay.[[ISODate]], MONTH-DAY).
+    auto fields = iso_date_to_fields(calendar, month_day->iso_date(), DateType::MonthDay);
+
+    // 6. Let inputFields be ? PrepareCalendarFields(calendar, item, « YEAR », « », « »).
+    auto input_fields = TRY(prepare_calendar_fields(vm, calendar, item.as_object(), { { CalendarField::Year } }, {}, CalendarFieldList {}));
+
+    // 7. Let mergedFields be CalendarMergeFields(calendar, fields, inputFields).
+    auto merged_fields = calendar_merge_fields(calendar, fields, input_fields);
+
+    // 8. Let isoDate be ? CalendarDateFromFields(calendar, mergedFields, CONSTRAIN).
+    auto iso_date = TRY(calendar_date_from_fields(vm, calendar, merged_fields, Overflow::Constrain));
+
+    // 9. Return ! CreateTemporalDate(isoDate, calendar).
+    return MUST(create_temporal_date(vm, iso_date, calendar));
+}
+
 }
diff --git a/Libraries/LibJS/Runtime/Temporal/PlainMonthDayPrototype.h b/Libraries/LibJS/Runtime/Temporal/PlainMonthDayPrototype.h
index 2780f77aa8c..ef74ed35113 100644
--- a/Libraries/LibJS/Runtime/Temporal/PlainMonthDayPrototype.h
+++ b/Libraries/LibJS/Runtime/Temporal/PlainMonthDayPrototype.h
@@ -32,6 +32,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(to_locale_string);
     JS_DECLARE_NATIVE_FUNCTION(to_json);
     JS_DECLARE_NATIVE_FUNCTION(value_of);
+    JS_DECLARE_NATIVE_FUNCTION(to_plain_date);
 };
 
 }
diff --git a/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.cpp b/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.cpp
index d1130e1f962..7f818b1c0f4 100644
--- a/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.cpp
+++ b/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.cpp
@@ -8,6 +8,7 @@
 #include <LibJS/Runtime/Temporal/AbstractOperations.h>
 #include <LibJS/Runtime/Temporal/Calendar.h>
 #include <LibJS/Runtime/Temporal/Duration.h>
+#include <LibJS/Runtime/Temporal/PlainDate.h>
 #include <LibJS/Runtime/Temporal/PlainYearMonthPrototype.h>
 
 namespace JS::Temporal {
@@ -51,6 +52,7 @@ void PlainYearMonthPrototype::initialize(Realm& realm)
     define_native_function(realm, vm.names.toLocaleString, to_locale_string, 0, attr);
     define_native_function(realm, vm.names.toJSON, to_json, 0, attr);
     define_native_function(realm, vm.names.valueOf, value_of, 0, attr);
+    define_native_function(realm, vm.names.toPlainDate, to_plain_date, 1, attr);
 }
 
 // 9.3.3 get Temporal.PlainYearMonth.prototype.calendarId, https://tc39.es/proposal-temporal/#sec-get-temporal.plainyearmonth.prototype.calendarid
@@ -297,4 +299,38 @@ JS_DEFINE_NATIVE_FUNCTION(PlainYearMonthPrototype::value_of)
     return vm.throw_completion<TypeError>(ErrorType::Convert, "Temporal.PlainYearMonth", "a primitive value");
 }
 
+// 9.3.23 Temporal.PlainYearMonth.prototype.toPlainDate ( item ), https://tc39.es/proposal-temporal/#sec-temporal.plainyearmonth.prototype.toplaindate
+JS_DEFINE_NATIVE_FUNCTION(PlainYearMonthPrototype::to_plain_date)
+{
+    auto item = vm.argument(0);
+
+    // 1. Let yearMonth be the this value.
+    // 2. Perform ? RequireInternalSlot(yearMonth, [[InitializedTemporalYearMonth]]).
+    auto year_month = TRY(typed_this_object(vm));
+
+    // 3. If item is not an Object, then
+    if (!item.is_object()) {
+        // a. Throw a TypeError exception.
+        return vm.throw_completion<TypeError>(ErrorType::NotAnObject, item);
+    }
+
+    // 4. Let calendar be yearMonth.[[Calendar]].
+    auto const& calendar = year_month->calendar();
+
+    // 5. Let fields be ISODateToFields(calendar, yearMonth.[[ISODate]], YEAR-MONTH).
+    auto fields = iso_date_to_fields(calendar, year_month->iso_date(), DateType::YearMonth);
+
+    // 6. Let inputFields be ? PrepareCalendarFields(calendar, item, « DAY », « », « »).
+    auto input_fields = TRY(prepare_calendar_fields(vm, calendar, item.as_object(), { { CalendarField::Day } }, {}, CalendarFieldList {}));
+
+    // 7. Let mergedFields be CalendarMergeFields(calendar, fields, inputFields).
+    auto merged_fields = calendar_merge_fields(calendar, fields, input_fields);
+
+    // 8. Let isoDate be ? CalendarDateFromFields(calendar, mergedFields, CONSTRAIN).
+    auto iso_date = TRY(calendar_date_from_fields(vm, calendar, merged_fields, Overflow::Constrain));
+
+    // 9. Return ! CreateTemporalDate(isoDate, calendar).
+    return MUST(create_temporal_date(vm, iso_date, calendar));
+}
+
 }
diff --git a/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.h b/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.h
index 30a9fd1ddbe..af3e72cd7f5 100644
--- a/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.h
+++ b/Libraries/LibJS/Runtime/Temporal/PlainYearMonthPrototype.h
@@ -43,6 +43,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(to_locale_string);
     JS_DECLARE_NATIVE_FUNCTION(to_json);
     JS_DECLARE_NATIVE_FUNCTION(value_of);
+    JS_DECLARE_NATIVE_FUNCTION(to_plain_date);
 };
 
 }
diff --git a/Libraries/LibJS/Tests/builtins/Temporal/PlainMonthDay/PlainMonthDay.prototype.toPlainDate.js b/Libraries/LibJS/Tests/builtins/Temporal/PlainMonthDay/PlainMonthDay.prototype.toPlainDate.js
new file mode 100644
index 00000000000..eb4942a9041
--- /dev/null
+++ b/Libraries/LibJS/Tests/builtins/Temporal/PlainMonthDay/PlainMonthDay.prototype.toPlainDate.js
@@ -0,0 +1,27 @@
+describe("normal behavior", () => {
+    test("length is 1", () => {
+        expect(Temporal.PlainMonthDay.prototype.toPlainDate).toHaveLength(1);
+    });
+
+    test("basic functionality", () => {
+        const plainMonthDay = new Temporal.PlainMonthDay(7, 6);
+        const plainDate = plainMonthDay.toPlainDate({ year: 2021 });
+        expect(plainDate.equals(new Temporal.PlainDate(2021, 7, 6))).toBeTrue();
+    });
+});
+
+describe("errors", () => {
+    test("argument must be an object", () => {
+        const plainMonthDay = new Temporal.PlainMonthDay(7, 6);
+        expect(() => {
+            plainMonthDay.toPlainDate(42);
+        }).toThrowWithMessage(TypeError, "42 is not an object");
+    });
+
+    test("year field is required", () => {
+        const plainMonthDay = new Temporal.PlainMonthDay(7, 6);
+        expect(() => {
+            plainMonthDay.toPlainDate({});
+        }).toThrowWithMessage(TypeError, "Required property year is missing or undefined");
+    });
+});
diff --git a/Libraries/LibJS/Tests/builtins/Temporal/PlainYearMonth/PlainYearMonth.prototype.toPlainDate.js b/Libraries/LibJS/Tests/builtins/Temporal/PlainYearMonth/PlainYearMonth.prototype.toPlainDate.js
new file mode 100644
index 00000000000..cf0f945da7b
--- /dev/null
+++ b/Libraries/LibJS/Tests/builtins/Temporal/PlainYearMonth/PlainYearMonth.prototype.toPlainDate.js
@@ -0,0 +1,27 @@
+describe("normal behavior", () => {
+    test("length is 1", () => {
+        expect(Temporal.PlainYearMonth.prototype.toPlainDate).toHaveLength(1);
+    });
+
+    test("basic functionality", () => {
+        const plainYearMonth = new Temporal.PlainYearMonth(2021, 7);
+        const plainDate = plainYearMonth.toPlainDate({ day: 6 });
+        expect(plainDate.equals(new Temporal.PlainDate(2021, 7, 6))).toBeTrue();
+    });
+});
+
+describe("errors", () => {
+    test("argument must be an object", () => {
+        const plainYearMonth = new Temporal.PlainYearMonth(2021, 7);
+        expect(() => {
+            plainYearMonth.toPlainDate(42);
+        }).toThrowWithMessage(TypeError, "42 is not an object");
+    });
+
+    test("day field is required", () => {
+        const plainYearMonth = new Temporal.PlainYearMonth(2021, 7);
+        expect(() => {
+            plainYearMonth.toPlainDate({});
+        }).toThrowWithMessage(TypeError, "Required property day is missing or undefined");
+    });
+});