Polyfill: Update Meiji era start date

Update internal era data to change hasYearZero to startingYear, so that
we can accommodate the starting year of 6.

Author: ptomato

polyfill/lib/calendar.mjs

@@ -767,12 +767,12 @@ const nonIsoHelperBase = {
           }
           // last era always gets all "leftover" (older than epoch) years,
           // so no need for a comparison like below.
-          eraYear = year - e.anchorEpoch.year + (e.hasYearZero ? 0 : 1);
+          eraYear = year - e.anchorEpoch.year + e.startingYear;
           return true;
         }
         const comparison = nonIsoHelperBase.compareCalendarDates(calendarDate, e.anchorEpoch);
         if (comparison >= 0) {
-          eraYear = year - e.anchorEpoch.year + (e.hasYearZero ? 0 : 1);
+          eraYear = year - e.anchorEpoch.year + e.startingYear;
           return true;
         }
         return false;
@@ -803,7 +803,7 @@ const nonIsoHelperBase = {
       if (matchingEra.reverseOf) {
         year = matchingEra.anchorEpoch.year - eraYear;
       } else {
-        year = eraYear + matchingEra.anchorEpoch.year - (matchingEra.hasYearZero ? 0 : 1);
+        year = eraYear + matchingEra.anchorEpoch.year - matchingEra.startingYear;
       }
       // We'll accept dates where the month/day is earlier than the start of
       // the era or after its end as long as it's in the same year. If that
@@ -1577,9 +1577,9 @@ const helperIndian = makeNonISOHelper([{ code: 'shaka', isoEpoch: { year: 79, mo
  *   // the last (oldest) era in the array.
  *   reverseOf?: Era;
  *
- *   // If true, the era's years are 0-based. If omitted or false,
- *   // then the era's years are 1-based.
- *   hasYearZero?: boolean;
+ *   // Number of the era's starting year (for example, 0 if there's a Year 0).
+ *   // If omitted, the era's years are 1-based.
+ *   startingYear?: number = 1;
  *
  *   // Override if this era is the anchor. Not normally used because
  *   // anchor eras are inferred.
@@ -1601,10 +1601,11 @@ function adjustEras(eras) {
   let anchorEra;
   Call(ArrayPrototypeForEach, eras, [
     (e) => {
+      e.startingYear ??= 1;
       if (e.isAnchor || (!e.anchorEpoch && !e.reverseOf)) {
         assert(!anchorEra, 'Invalid era data: cannot have multiple anchor eras');
         anchorEra = e;
-        e.anchorEpoch = { year: e.hasYearZero ? 0 : 1 };
+        e.anchorEpoch = { year: e.startingYear };
       } else {
         assert(e.code, 'Invalid era data: if era name is blank, it must be the anchor era');
       }
@@ -1701,7 +1702,7 @@ const makeHelperGregorian = (id, originalEras) => {
       calendarDate = this.adjustCalendarDate(calendarDate);
       const { year, month, day } = calendarDate;
       const { anchorEra } = this;
-      const isoYearEstimate = year + anchorEra.isoEpoch.year - (anchorEra.hasYearZero ? 0 : 1);
+      const isoYearEstimate = year + anchorEra.isoEpoch.year - anchorEra.startingYear;
       return ES.RegulateISODate(isoYearEstimate, month, day, 'constrain');
     }
   });
@@ -1821,18 +1822,18 @@ const helperGregory = ObjectAssign(
 
 const helperJapanese = ObjectAssign(
   // NOTE: Only the 5 modern eras (Meiji and later) are included. For dates
-  // before Meiji 1, the `ce` and `bce` eras are used. Challenges with pre-Meiji
+  // before Meiji 6, the `ce` and `bce` eras are used. Challenges with pre-Meiji
   // eras include:
   // - Start/end dates of older eras are not precisely defined, which is
   //   challenging given Temporal's need for precision
   // - Some era dates and/or names are disputed by historians
   // - As historical research proceeds, new eras are discovered and existing era
   //   dates are modified, leading to considerable churn which is not good for
   //   Temporal use.
-  //  - The earliest era (in 645 CE) may not end up being the earliest depending
-  //    on future historical scholarship
-  //  - Before Meiji, Japan used a lunar (or lunisolar?) calendar but AFAIK
-  //    that's not reflected in the ICU implementation.
+  // - The earliest era (in 645 CE) may not end up being the earliest depending
+  //   on future historical scholarship
+  // - Before Meiji, Japan used a lunisolar calendar but that's not reflected in
+  //   the ICU implementation.
   //
   // For more discussion: https://github.com/tc39/proposal-temporal/issues/526.
   //
@@ -1842,21 +1843,16 @@ const helperJapanese = ObjectAssign(
   //
   // NOTE: Japan started using the Gregorian calendar in 6 Meiji, replacing a
   // lunisolar calendar. So the day before January 1 of 6 Meiji (1873) was not
-  // December 31, but December 2, of 5 Meiji (1872). The existing Ecma-402
-  // Japanese calendar doesn't seem to take this into account, so neither do we:
-  // > args = ['en-ca-u-ca-japanese', { era: 'short' }]
-  // > new Date('1873-01-01T12:00').toLocaleString(...args)
-  // '1 1, 6 Meiji, 12:00:00 PM'
-  // > new Date('1872-12-31T12:00').toLocaleString(...args)
-  // '12 31, 5 Meiji, 12:00:00 PM'
+  // December 31, but M12-02, of 5 Meiji (1872). To avoid confusion between
+  // lunisolar and solar dates we count years prior to 1872 as CE.
   makeHelperSameMonthDayAsGregorian('japanese', [
     // The Japanese calendar `year` is just the ISO year, because (unlike other
     // ICU calendars) there's no obvious "default era", we use the ISO year.
     { code: 'reiwa', isoEpoch: { year: 2019, month: 5, day: 1 }, anchorEpoch: { year: 2019, month: 5, day: 1 } },
     { code: 'heisei', isoEpoch: { year: 1989, month: 1, day: 8 }, anchorEpoch: { year: 1989, month: 1, day: 8 } },
     { code: 'showa', isoEpoch: { year: 1926, month: 12, day: 25 }, anchorEpoch: { year: 1926, month: 12, day: 25 } },
     { code: 'taisho', isoEpoch: { year: 1912, month: 7, day: 30 }, anchorEpoch: { year: 1912, month: 7, day: 30 } },
-    { code: 'meiji', isoEpoch: { year: 1868, month: 10, day: 23 }, anchorEpoch: { year: 1868, month: 10, day: 23 } },
+    { code: 'meiji', isoEpoch: { year: 1873, month: 1, day: 1 }, anchorEpoch: { year: 1873 }, startingYear: 6 },
     { code: 'ce', isoEpoch: { year: 1, month: 1, day: 1 } },
     { code: 'bce', reverseOf: 'ce' }
   ]),