Add test for skipped initial days in Temporal.Calendar.prototype.year…

…MonthFromFields

test/intl402/Temporal/Calendar/prototype/yearMonthFromFields/reference-day-japanese.js

@@ -0,0 +1,47 @@
+// Copyright (C) 2024 André Bargull. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.calendar.prototype.yearmonthfromfields
+description: >
+  Reference ISO day is chosen to be the first day of the calendar month.
+info: |
+  7.c. Let firstDayIndex be the 1-based index of the first day of the month
+       described by fields (i.e., 1 unless the month's first day is skipped by
+       this calendar.)
+    d. Perform ! CreateDataPropertyOrThrow(fields, "day", 𝔽(firstDayIndex)).
+includes: [temporalHelpers.js]
+features: [Temporal]
+---*/
+
+// Notes:
+// - "heisei" era started January 8, 1989.
+// - "reiwa" era started May 1, 2019.
+
+const japanese = new Temporal.Calendar("japanese");
+
+const result1 = japanese.yearMonthFromFields({ year: 2024, monthCode: "M01" });
+TemporalHelpers.assertPlainYearMonth(
+  result1,
+  2024, 1, "M01",
+  "reference day is 1",
+  "reiwa", 6, /* reference day = */ 1
+);
+
+// Reiwa era started on the first day of the month, so the reference day is 1.
+const result2 = japanese.yearMonthFromFields({ era: "reiwa", eraYear: 1, monthCode: "M05" });
+TemporalHelpers.assertPlainYearMonth(
+  result2,
+  2019, 5, "M05",
+  "reference day is 1",
+  "reiwa", 1, /* reference day = */ 1
+);
+
+// Heisei era started on the eighth day of the month, so the reference day is 8.
+const result3 = japanese.yearMonthFromFields({ era: "heisei", eraYear: 1, monthCode: "M01" });
+TemporalHelpers.assertPlainYearMonth(
+  result3,
+  1989, 1, "M01",
+  "reference day is 8",
+  "heisei", 1, /* reference day = */ 8
+);