Normative: Treat daysInMonth as count of all days rather than numbe…

…r of the last day Updates PlainYearMonth arithmetic accordingly and clarifies language around non-ISO calendar operations. Fixes #1315 Co-authored-by: Guillaume Emont <[email protected]>
tc39 · Apr 28, 2023 · eb2a26f · eb2a26f
1 parent d1161a1
commit eb2a26f
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Showing 2 changed files with 28 additions and 7 deletions.
diff --git a/polyfill/lib/ecmascript.mjs b/polyfill/lib/ecmascript.mjs
@@ -4735,13 +4735,24 @@ export function AddDurationToOrSubtractDurationFromPlainYearMonth(operation, yea
   const calendar = GetSlot(yearMonth, CALENDAR);
   const fieldNames = CalendarFields(calendar, ['monthCode', 'year']);
   const fields = PrepareTemporalFields(yearMonth, fieldNames, []);
+  const fieldsCopy = ObjectCreate(null);
+  CopyDataProperties(fieldsCopy, fields, []);
+  fields.day = 1;
+  let startDate = CalendarDateFromFields(calendar, fields);
   const sign = DurationSign(years, months, weeks, days, 0, 0, 0, 0, 0, 0);
-  fields.day = sign < 0 ? CalendarDaysInMonth(calendar, yearMonth) : 1;
-  const startDate = CalendarDateFromFields(calendar, fields);
+  const dateAdd = GetMethod(calendar, 'dateAdd');
   const Duration = GetIntrinsic('%Temporal.Duration%');
+  if (sign < 0) {
+    const oneMonthDuration = new Duration(0, 1, 0, 0, 0, 0, 0, 0, 0, 0);
+    const nextMonth = CalendarDateAdd(calendar, startDate, oneMonthDuration, undefined, dateAdd);
+    const minusDayDuration = new Duration(0, 0, 0, -1, 0, 0, 0, 0, 0, 0);
+    const endOfMonth = CalendarDateAdd(calendar, nextMonth, minusDayDuration, undefined, dateAdd);
+    fieldsCopy.day = CalendarDay(calendar, endOfMonth);
+    startDate = CalendarDateFromFields(calendar, fieldsCopy);
+  }
   const durationToAdd = new Duration(years, months, weeks, days, 0, 0, 0, 0, 0, 0);
   const optionsCopy = CopyOptions(options);
-  const addedDate = CalendarDateAdd(calendar, startDate, durationToAdd, options);
+  const addedDate = CalendarDateAdd(calendar, startDate, durationToAdd, options, dateAdd);
   const addedDateFields = PrepareTemporalFields(addedDate, fieldNames, []);
 
   return CalendarYearMonthFromFields(calendar, addedDateFields, optionsCopy);

diff --git a/spec/plainyearmonth.html b/spec/plainyearmonth.html
@@ -666,16 +666,26 @@ <h1>
         1. Let _calendar_ be _yearMonth_.[[Calendar]].
         1. Let _fieldNames_ be ? CalendarFields(_calendar_, « *"monthCode"*, *"year"* »).
         1. Let _fields_ be ? PrepareTemporalFields(_yearMonth_, _fieldNames_, «»).
+        1. Let _fieldsCopy_ be OrdinaryObjectCreate(*null*).
+        1. Perform ! CopyDataProperties(_fieldsCopy_, _fields_, «»).
+        1. Perform ! CreateDataPropertyOrThrow(_fields_, *"day"*, *1*<sub>𝔽</sub>).
+        1. Let _intermediateDate_ be ? CalendarDateFromFields(_calendar_, _fields_).
         1. Let _sign_ be ! DurationSign(_duration_.[[Years]], _duration_.[[Months]], _duration_.[[Weeks]], _balanceResult_.[[Days]], 0, 0, 0, 0, 0, 0).
+        1. Let _dateAdd_ be ? GetMethod(_calendar_, *"dateAdd"*).
         1. If _sign_ &lt; 0, then
-          1. Let _day_ be ? CalendarDaysInMonth(_calendar_, _yearMonth_).
+          1. Let _oneMonthDuration_ be ! CreateTemporalDuration(0, 1, 0, 0, 0, 0, 0, 0, 0, 0).
+          1. Let _nextMonth_ be ? CalendarDateAdd(_calendar_, _intermediateDate_, _oneMonthDuration_, *undefined*, _dateAdd_).
+          1. Let _minusDayDuration_ be ! CreateTemporalDuration(0, 0, 0, -1, 0, 0, 0, 0, 0, 0).
+          1. Let _endOfMonth_ be ? CalendarDateAdd(_calendar_, _nextMonth_, _minusDayDuration_, *undefined*, _dateAdd_).
+          1. Let _day_ be ? CalendarDay(_calendar_, _endOfMonth_).
+          1. Perform ! CreateDataPropertyOrThrow(_fieldsCopy_, *"day"*, _day_).
+          1. Let _date_ be ? CalendarDateFromFields(_calendar_, _fieldsCopy_).
         1. Else,
-          1. Let _day_ be 1.
-        1. Perform ! CreateDataPropertyOrThrow(_fields_, *"day"*, 𝔽(_day_)).
-        1. Let _date_ be ? CalendarDateFromFields(_calendar_, _fields_).
+          1. Let _date_ be _intermediateDate_.
         1. Let _durationToAdd_ be ! CreateTemporalDuration(_duration_.[[Years]], _duration_.[[Months]], _duration_.[[Weeks]], _balanceResult_.[[Days]], 0, 0, 0, 0, 0, 0).
         1. Let _optionsCopy_ be ? CopyOptions(_options_).
         1. Let _addedDate_ be ? CalendarDateAdd(_calendar_, _date_, _durationToAdd_, _options_).
+        1. Let _addedDate_ be ? CalendarDateAdd(_calendar_, _date_, _durationToAdd_, _options_, _dateAdd_).
         1. Let _addedDateFields_ be ? PrepareTemporalFields(_addedDate_, _fieldNames_, «»).
         1. Return ? CalendarYearMonthFromFields(_calendar_, _addedDateFields_, _optionsCopy_).
       </emu-alg>