diff --git a/polyfill/lib/calendar.mjs b/polyfill/lib/calendar.mjs
index a494a7bf94..cbc93ee33e 100644
--- a/polyfill/lib/calendar.mjs
+++ b/polyfill/lib/calendar.mjs
@@ -324,9 +324,6 @@ impl['iso8601'] = {
   monthDayFromFields(fields, options, calendarSlotValue) {
     fields = ES.PrepareTemporalFields(fields, ['day', 'month', 'monthCode', 'year'], ['day']);
     const overflow = ES.ToTemporalOverflow(options);
-    if (fields.month !== undefined && fields.year === undefined && fields.monthCode === undefined) {
-      throw new TypeError('either year or monthCode required with month');
-    }
     const referenceISOYear = 1972;
     fields = resolveNonLunisolarMonth(fields);
     let { month, day, year } = fields;
@@ -701,7 +698,7 @@ const nonIsoHelperBase = {
     }
     if (this.hasEra) {
       if ((calendarDate['era'] === undefined) !== (calendarDate['eraYear'] === undefined)) {
-        throw new RangeError("properties 'era' and 'eraYear' must be provided together");
+        throw new TypeError("properties 'era' and 'eraYear' must be provided together");
       }
     }
   },
diff --git a/polyfill/lib/ecmascript.mjs b/polyfill/lib/ecmascript.mjs
index 9fa10ac025..03fa2336b6 100644
--- a/polyfill/lib/ecmascript.mjs
+++ b/polyfill/lib/ecmascript.mjs
@@ -1345,24 +1345,16 @@ export function ToTemporalMonthDay(item, options) {
   if (options !== undefined) options = SnapshotOwnProperties(GetOptionsObject(options), null);
   if (Type(item) === 'Object') {
     if (IsTemporalMonthDay(item)) return item;
-    let calendar, calendarAbsent;
+    let calendar;
     if (HasSlot(item, CALENDAR)) {
       calendar = GetSlot(item, CALENDAR);
-      calendarAbsent = false;
     } else {
       calendar = item.calendar;
-      calendarAbsent = calendar === undefined;
       if (calendar === undefined) calendar = 'iso8601';
       calendar = ToTemporalCalendarSlotValue(calendar);
     }
     const fieldNames = CalendarFields(calendar, ['day', 'month', 'monthCode', 'year']);
     const fields = PrepareTemporalFields(item, fieldNames, []);
-    // Callers who omit the calendar are not writing calendar-independent
-    // code. In that case, `monthCode`/`year` can be omitted; `month` and
-    // `day` are sufficient. Add a `year` to satisfy calendar validation.
-    if (calendarAbsent && fields.month !== undefined && fields.monthCode === undefined && fields.year === undefined) {
-      fields.year = 1972;
-    }
     return CalendarMonthDayFromFields(calendar, fields, options);
   }
 
@@ -1373,6 +1365,9 @@ export function ToTemporalMonthDay(item, options) {
   ToTemporalOverflow(options); // validate and ignore
 
   if (referenceISOYear === undefined) {
+    if (calendar !== 'iso8601') {
+      throw new Error(`assertion failed: missing year with non-"iso8601" calendar identifier ${calendar}`);
+    }
     RejectISODate(1972, month, day);
     return CreateTemporalMonthDay(month, day, calendar);
   }
diff --git a/polyfill/test262 b/polyfill/test262
index dceb204259..892a5dccd2 160000
--- a/polyfill/test262
+++ b/polyfill/test262
@@ -1 +1 @@
-Subproject commit dceb204259abc800b7a6ec7bccf07f9a7c69297d
+Subproject commit 892a5dccd2a59a63f3a363db07f53bf896912699
diff --git a/spec/calendar.html b/spec/calendar.html
index 8b153b8f7d..23653f10af 100644
--- a/spec/calendar.html
+++ b/spec/calendar.html
@@ -917,7 +917,7 @@ <h1>ISOMonthDayFromFields ( _fields_, _overflow_ )</h1>
         1. Let _day_ be ! Get(_fields_, *"day"*).
         1. Assert: _month_ and _day_ are Numbers.
         1. Let _year_ be ! Get(_fields_, *"year"*).
-        1. Let _referenceISOYear_ be 1972 (the first leap year after the epoch).
+        1. Let _referenceISOYear_ be 1972 (the first ISO 8601 leap year after the epoch).
         1. If _year_ is *undefined*, then
           1. Let _result_ be ? RegulateISODate(_referenceISOYear_, ℝ(_month_), ℝ(_day_), _overflow_).
         1. Else,
diff --git a/spec/intl.html b/spec/intl.html
index 1ac51aef19..2b5ed4a7af 100644
--- a/spec/intl.html
+++ b/spec/intl.html
@@ -1855,6 +1855,7 @@ <h1>
           <ul>
             <li>If _type_ is ~date~ or ~month-day~ and *"day"* in the calendar has an interpretation analogous to ISO 8601 and its corresponding value is *undefined*.</li>
             <li>If *"month"* and *"monthCode"* in the calendar have interpretations analogous to ISO 8601 and either the corresponding values for both are *undefined* or neither value is *undefined* but they do not identify the same month.</li>
+            <li>If _type_ is ~month-day~ and the value for *"monthCode"* is *undefined* and a year cannot be determined from _fields_.</li>
             <li>If _type_ is ~date~ or ~year-month~ and the calendar supports the usual partitioning of years into eras with their own year counting as represented by *"year"*, *"era"*, and *"eraYear"* (as in the Gregorian or traditional Japanese calendars) and any of the following cases apply:<ul>
               <li>The value for each of *"year"* and *"era"* and *"eraYear"* is *undefined*.</li>
               <li>The value for *"era"* is *undefined* but the value for *"eraYear"* is not.</li>
@@ -1862,6 +1863,9 @@ <h1>
               <li>None of the three values are *undefined* but the values for *"era"* and *"eraYear"* do not together identify the same year as the value for *"year"*.</li>
             </ul></li>
           </ul>
+          <emu-note>
+            When _type_ is ~month-day~ and *"month"* is provided without *"monthCode"*, it is recommended that all built-in calendars other than the ISO 8601 calendar require a disambiguating year (e.g., either *"year"* or *"era"* and *"eraYear"*), including those that always use exactly the same months as the ISO 8601 calendar (which receives special handling in this specification as a default calendar that is permanently stable for automated processing).
+          </emu-note>
         </emu-clause>
       </emu-clause>
 
diff --git a/spec/plainmonthday.html b/spec/plainmonthday.html
index 44a2ecf6b4..57d9aa874a 100644
--- a/spec/plainmonthday.html
+++ b/spec/plainmonthday.html
@@ -32,14 +32,13 @@ <h1>Temporal.PlainMonthDay ( _isoMonth_, _isoDay_ [ , _calendarLike_ [ , _refere
         1. If NewTarget is *undefined*, then
           1. Throw a *TypeError* exception.
         1. If _referenceISOYear_ is *undefined*, then
-          1. Set _referenceISOYear_ to *1972*<sub>𝔽</sub>.
+          1. Set _referenceISOYear_ to *1972*<sub>𝔽</sub> (the first ISO 8601 leap year after the epoch).
         1. Let _m_ be ? ToIntegerWithTruncation(_isoMonth_).
         1. Let _d_ be ? ToIntegerWithTruncation(_isoDay_).
         1. Let _calendar_ be ? ToTemporalCalendarSlotValue(_calendarLike_, *"iso8601"*).
-        1. Let _ref_ be ? ToIntegerWithTruncation(_referenceISOYear_).
-        1. Return ? CreateTemporalMonthDay(_m_, _d_, _calendar_, _ref_, NewTarget).
+        1. Let _y_ be ? ToIntegerWithTruncation(_referenceISOYear_).
+        1. Return ? CreateTemporalMonthDay(_m_, _d_, _calendar_, _y_, NewTarget).
       </emu-alg>
-      <emu-note>1972 is the first leap year after the epoch.</emu-note>
     </emu-clause>
   </emu-clause>
 
@@ -366,27 +365,16 @@ <h1>
       <emu-alg>
         1. If _options_ is not present, set _options_ to *undefined*.
         1. If _options_ is not *undefined*, set _options_ to ? SnapshotOwnProperties(! GetOptionsObject(_options_), *null*).
-        1. Let _referenceISOYear_ be 1972 (the first leap year after the epoch).
         1. If Type(_item_) is Object, then
           1. If _item_ has an [[InitializedTemporalMonthDay]] internal slot, then
             1. Return _item_.
           1. If _item_ has an [[InitializedTemporalDate]], [[InitializedTemporalDateTime]], [[InitializedTemporalYearMonth]], or [[InitializedTemporalZonedDateTime]] internal slot, then
             1. Let _calendar_ be _item_.[[Calendar]].
-            1. Let _calendarAbsent_ be *false*.
           1. Else,
             1. Let _calendarLike_ be ? Get(_item_, *"calendar"*).
-            1. If _calendarLike_ is *undefined*, then
-              1. Let _calendarAbsent_ be *true*.
-            1. Else,
-              1. Let _calendarAbsent_ be *false*.
             1. Let _calendar_ be ? ToTemporalCalendarSlotValue(_calendarLike_, *"iso8601"*).
           1. Let _fieldNames_ be ? CalendarFields(_calendar_, « *"day"*, *"month"*, *"monthCode"*, *"year"* »).
           1. Let _fields_ be ? PrepareTemporalFields(_item_, _fieldNames_, «»).
-          1. Let _month_ be ! Get(_fields_, *"month"*).
-          1. Let _monthCode_ be ! Get(_fields_, *"monthCode"*).
-          1. Let _year_ be ! Get(_fields_, *"year"*).
-          1. If _calendarAbsent_ is *true*, and _month_ is not *undefined*, and _monthCode_ is *undefined* and _year_ is *undefined*, then
-            1. Perform ! CreateDataPropertyOrThrow(_fields_, *"year"*, 𝔽(_referenceISOYear_)).
           1. Return ? CalendarMonthDayFromFields(_calendar_, _fields_, _options_).
         1. If _item_ is not a String, throw a *TypeError* exception.
         1. Let _result_ be ? ParseTemporalMonthDayString(_item_).
@@ -396,6 +384,8 @@ <h1>
         1. Set _calendar_ to the ASCII-lowercase of _calendar_.
         1. Perform ? ToTemporalOverflow(_options_).
         1. If _result_.[[Year]] is *undefined*, then
+          1. Assert: _calendar_ is *"iso8601"*.
+          1. Let _referenceISOYear_ be 1972 (the first ISO 8601 leap year after the epoch).
           1. Return ? CreateTemporalMonthDay(_result_.[[Month]], _result_.[[Day]], _calendar_, _referenceISOYear_).
         1. Set _result_ to ? CreateTemporalMonthDay(_result_.[[Month]], _result_.[[Day]], _calendar_, _result_.[[Year]]).
         1. NOTE: The following operation is called without _options_, in order for the calendar to store a canonical value in the [[ISOYear]] internal slot of the result.