Add eras to Gregorian calendar

As part of this issue we decided that another difference between the ISO
and Gregorian calendars is that the latter has BC/AD eras. It's
indeterminate how this will exactly be standardized, as that's up to
ECMA-402, but we add it to the experimental Gregorian calendar in the
polyfill.

See: #901

polyfill/lib/calendar.mjs

@@ -283,13 +283,51 @@ class ISO8601Calendar extends Calendar {
 
 MakeIntrinsicClass(ISO8601Calendar, 'Temporal.ISO8601Calendar');
 
-// According to documentation for Intl.Locale.prototype.calendar on MDN,
+// Note: other built-in calendars than iso8601 are not part of the Temporal
+// proposal for ECMA-262. These calendars will be standardized as part of
+// ECMA-402.
+
+// Implementation details for Gregorian calendar
+const gre = {
+  isoYear(year, era) {
+    return era === 'bc' ? -(year - 1) : year;
+  }
+};
+
 // 'iso8601' calendar is equivalent to 'gregory' except for ISO 8601 week
-// numbering rules, which we do not currently use in Temporal.
+// numbering rules, which we do not currently use in Temporal, and the addition
+// of BC/AD eras which means no negative years or year 0.
 class Gregorian extends ISO8601Calendar {
   constructor() {
     super('gregory');
   }
+
+  era(date) {
+    return GetSlot(date, ISO_YEAR) < 1 ? 'bc' : 'ad';
+  }
+  year(date) {
+    const isoYear = GetSlot(date, ISO_YEAR);
+    return isoYear < 1 ? -isoYear + 1 : isoYear;
+  }
+
+  fields(fields) {
+    fields = super.fields(fields);
+    if (fields.includes('year')) fields.push('era');
+    return fields;
+  }
+
+  dateFromFields(fields, options, constructor) {
+    // Intentionally alphabetical
+    fields = ES.ToRecord(fields, [['day'], ['era', 'ad'], ['month'], ['year']]);
+    const isoYear = gre.isoYear(fields.year, fields.era);
+    return super.dateFromFields({ ...fields, year: isoYear }, options, constructor);
+  }
+  yearMonthFromFields(fields, options, constructor) {
+    // Intentionally alphabetical
+    fields = ES.ToRecord(fields, [['era', 'ad'], ['month'], ['year']]);
+    const isoYear = gre.isoYear(fields.year, fields.era);
+    return super.yearMonthFromFields({ ...fields, year: isoYear }, options, constructor);
+  }
 }
 
 // Implementation details for Japanese calendar

polyfill/test/calendar.mjs

@@ -150,6 +150,32 @@ describe('Calendar', () => {
     });
   });
 });
+describe('Built-in calendars (not standardized yet)', () => {
+  describe('gregory', () => {
+    it('era AD', () => {
+      const date = Temporal.Date.from('1999-12-31[c=gregory]');
+      equal(date.era, 'ad');
+      equal(date.year, 1999);
+    });
+    it('era BC', () => {
+      const date = Temporal.Date.from('-000001-12-31[c=gregory]');
+      equal(date.era, 'bc');
+      equal(date.year, 2);
+    });
+    it('can create from fields with era AD', () => {
+      const date = Temporal.Date.from({ era: 'ad', year: 1999, month: 12, day: 31, calendar: 'gregory' });
+      equal(`${date}`, '1999-12-31[c=gregory]');
+    });
+    it('era AD is the default', () => {
+      const date = Temporal.Date.from({ year: 1999, month: 12, day: 31, calendar: 'gregory' });
+      equal(`${date}`, '1999-12-31[c=gregory]');
+    });
+    it('can create from fields with era BC', () => {
+      const date = Temporal.Date.from({ era: 'bc', year: 2, month: 12, day: 31, calendar: 'gregory' });
+      equal(`${date}`, '-000001-12-31[c=gregory]');
+    });
+  });
+});
 
 import { normalize } from 'path';
 if (normalize(import.meta.url.slice(8)) === normalize(process.argv[1])) {