diff --git a/docs/calendar.md b/docs/calendar.md
index 9a30761ddc..771606ed3c 100644
--- a/docs/calendar.md
+++ b/docs/calendar.md
@@ -41,6 +41,8 @@ For specialized applications where you need to do calculations in a calendar sys
 To do this, create a class inheriting from `Temporal.Calendar`, call `super()` in the constructor with a calendar identifier, and implement all the methods.
 Any subclass of `Temporal.Calendar` will be accepted in Temporal APIs where a built-in `Temporal.Calendar` would work.
 
+The identifier of a custom calendar must not be any of the built-in calendar identifiers, and must consist of one or more components of between 2 and 8 ASCII alphanumeric characters each, separated by dashes, as described in [BCP 47](https://tools.ietf.org/html/bcp47#section-2.1).
+
 ### Protocol
 
 It's also possible for a plain object to be a custom calendar, without subclassing.
diff --git a/polyfill/lib/calendar.mjs b/polyfill/lib/calendar.mjs
index 7a33c30c38..3b623b50dd 100644
--- a/polyfill/lib/calendar.mjs
+++ b/polyfill/lib/calendar.mjs
@@ -2,10 +2,14 @@
 
 import { ES } from './ecmascript.mjs';
 import { GetIntrinsic, MakeIntrinsicClass, DefineIntrinsic } from './intrinsicclass.mjs';
+import * as REGEX from './regex.mjs';
 import { CALENDAR_ID, ISO_YEAR, ISO_MONTH, ISO_DAY, CreateSlots, GetSlot, SetSlot } from './slots.mjs';
 
+const ID_REGEX = new RegExp(`^${REGEX.calendarID.source}$`);
+
 export class Calendar {
   constructor(id) {
+    if (!ID_REGEX.exec(id)) throw new RangeError(`invalid calendar identifier ${id}`);
     CreateSlots(this);
     SetSlot(this, CALENDAR_ID, id);
 
diff --git a/polyfill/lib/regex.mjs b/polyfill/lib/regex.mjs
index 18d077344a..5c5c52c85b 100644
--- a/polyfill/lib/regex.mjs
+++ b/polyfill/lib/regex.mjs
@@ -1,9 +1,12 @@
+const calComponent = /[A-Za-z0-9]{2,8}/;
+export const calendarID = new RegExp(`(?:${calComponent.source}(?:-${calComponent.source})*)`);
+
 const yearpart = /(?:[+-\u2212]\d{6}|\d{4})/;
 const datesplit = new RegExp(`(${yearpart.source})(?:-(\\d{2})-(\\d{2})|(\\d{2})(\\d{2}))`);
 const timesplit = /(\d{2})(?::(\d{2})(?::(\d{2})(?:[.,](\d{1,9}))?)?|(\d{2})(?:(\d{2})(?:[.,](\d{1,9}))?)?)?/;
 export const offset = /([+-\u2212])([0-2][0-9])(?::?([0-5][0-9]))?/;
 const zonesplit = new RegExp(`(?:([zZ])|(?:${offset.source}?(?:\\[(?!c=)([^\\]\\s]*)?\\])?))`);
-const calendar = /\[c=([^\]\s]+)\]/;
+const calendar = new RegExp(`\\[c=(${calendarID.source})\\]`);
 
 export const instant = new RegExp(
   `^${datesplit.source}(?:T|\\s+)${timesplit.source}${zonesplit.source}(?:${calendar.source})?$`,
diff --git a/polyfill/test/instant.mjs b/polyfill/test/instant.mjs
index e75cc4555c..16942b9eff 100644
--- a/polyfill/test/instant.mjs
+++ b/polyfill/test/instant.mjs
@@ -356,7 +356,7 @@ describe('Instant', () => {
       equal(`${Instant.from('1976-11-18T15Z')}`, '1976-11-18T15:00Z');
     });
     it('ignores any specified calendar', () =>
-      equal(`${Instant.from('1976-11-18T15:23:30.123456789Z[c=discordian]')}`, '1976-11-18T15:23:30.123456789Z'));
+      equal(`${Instant.from('1976-11-18T15:23:30.123456789Z[c=discord]')}`, '1976-11-18T15:23:30.123456789Z'));
     it('no junk at end of string', () => throws(() => Instant.from('1976-11-18T15:23:30.123456789Zjunk'), RangeError));
   });
   describe('Instant.add works', () => {
diff --git a/polyfill/test/regex.mjs b/polyfill/test/regex.mjs
index 0c3b82bc5e..ae7e5293df 100644
--- a/polyfill/test/regex.mjs
+++ b/polyfill/test/regex.mjs
@@ -5,7 +5,7 @@ import Pretty from '@pipobscure/demitasse-pretty';
 const { reporter } = Pretty;
 
 import { strict as assert } from 'assert';
-const { equal } = assert;
+const { equal, throws } = assert;
 
 import * as Temporal from 'proposal-temporal';
 
@@ -534,6 +534,33 @@ describe('fromString regex', () => {
       }
     }
   });
+
+  describe('calendar ID', () => {
+    function makeCustomCalendar(id) {
+      return class extends Temporal.Calendar {
+        constructor() {
+          super(id);
+        }
+      };
+    }
+    describe('valid', () => {
+      ['aa', 'aa-aa', 'eightZZZ', 'eightZZZ-eightZZZ'].forEach((id) => {
+        it(id, () => {
+          const Custom = makeCustomCalendar(id);
+          const calendar = new Custom();
+          equal(calendar.id, id);
+        });
+      });
+    });
+    describe('not valid', () => {
+      ['a', 'a-a', 'foo_', 'foo.', 'ninechars', 'ninechars-ninechars'].forEach((id) => {
+        it(id, () => {
+          const Custom = makeCustomCalendar(id);
+          throws(() => new Custom(), RangeError);
+        });
+      });
+    });
+  });
 });
 
 import { normalize } from 'path';
diff --git a/polyfill/test/usercalendar.mjs b/polyfill/test/usercalendar.mjs
index 512d24b86c..12478bbaef 100644
--- a/polyfill/test/usercalendar.mjs
+++ b/polyfill/test/usercalendar.mjs
@@ -21,7 +21,7 @@ describe('Userland calendar', () => {
     const ISO8601Calendar = Temporal.Calendar.from('iso8601').constructor;
     class ZeroBasedCalendar extends ISO8601Calendar {
       constructor() {
-        super('zerobased');
+        super('zero-based');
       }
       dateFromFields(fields, options, constructor) {
         fields.month++;
@@ -47,13 +47,13 @@ describe('Userland calendar', () => {
     const md = Temporal.MonthDay.from({ month: 5, day: 5, calendar: obj });
 
     it('is a calendar', () => equal(typeof obj, 'object'));
-    it('.id property', () => equal(obj.id, 'zerobased'));
+    it('.id property', () => equal(obj.id, 'zero-based'));
     // FIXME: what should happen in Temporal.Calendar.from(obj)?
     it('.id is not available in from()', () => {
-      throws(() => Temporal.Calendar.from('zerobased'), RangeError);
-      throws(() => Temporal.Calendar.from('2020-06-05T09:34-07:00[America/Vancouver][c=zerobased]'), RangeError);
+      throws(() => Temporal.Calendar.from('zero-based'), RangeError);
+      throws(() => Temporal.Calendar.from('2020-06-05T09:34-07:00[America/Vancouver][c=zero-based]'), RangeError);
     });
-    it('Temporal.Date.from()', () => equal(`${date}`, '2020-06-05[c=zerobased]'));
+    it('Temporal.Date.from()', () => equal(`${date}`, '2020-06-05[c=zero-based]'));
     it('Temporal.Date fields', () => {
       equal(date.year, 2020);
       equal(date.month, 5);
@@ -67,7 +67,7 @@ describe('Userland calendar', () => {
       const date2 = Temporal.Date.from('2020-06-05');
       assert(date2.withCalendar(obj).equals(date));
     });
-    it('Temporal.DateTime.from()', () => equal(`${dt}`, '2020-06-05T12:00[c=zerobased]'));
+    it('Temporal.DateTime.from()', () => equal(`${dt}`, '2020-06-05T12:00[c=zero-based]'));
     it('Temporal.DateTime fields', () => {
       equal(dt.year, 2020);
       equal(dt.month, 5);
@@ -87,7 +87,7 @@ describe('Userland calendar', () => {
       const dt2 = Temporal.DateTime.from('2020-06-05T12:00');
       assert(dt2.withCalendar(obj).equals(dt));
     });
-    it('Temporal.YearMonth.from()', () => equal(`${ym}`, '2020-06-01[c=zerobased]'));
+    it('Temporal.YearMonth.from()', () => equal(`${ym}`, '2020-06-01[c=zero-based]'));
     it('Temporal.YearMonth fields', () => {
       equal(dt.year, 2020);
       equal(dt.month, 5);
@@ -96,7 +96,7 @@ describe('Userland calendar', () => {
       const ym2 = ym.with({ month: 0 });
       equal(ym2.month, 0);
     });
-    it('Temporal.MonthDay.from()', () => equal(`${md}`, '1972-06-05[c=zerobased]'));
+    it('Temporal.MonthDay.from()', () => equal(`${md}`, '1972-06-05[c=zero-based]'));
     it('Temporal.MonthDay fields', () => {
       equal(dt.month, 5);
       equal(dt.day, 5);
@@ -135,87 +135,87 @@ describe('Userland calendar', () => {
             id = `${item}`;
             // TODO: Use Temporal.parse here to extract the ID from an ISO string
           }
-          if (id === 'zerobased') return new ZeroBasedCalendar();
+          if (id === 'zero-based') return new ZeroBasedCalendar();
           return originalTemporalCalendarFrom.call(this, id);
         };
       });
       it('works for Calendar.from(id)', () => {
-        const tz = Temporal.Calendar.from('zerobased');
+        const tz = Temporal.Calendar.from('zero-based');
         assert(tz instanceof ZeroBasedCalendar);
       });
-      const iso = '1970-01-01T00:00+00:00[c=zerobased]';
+      const iso = '1970-01-01T00:00+00:00[c=zero-based]';
       it.skip('works for Calendar.from(ISO string)', () => {
         const tz = Temporal.Calendar.from(iso);
         assert(tz instanceof ZeroBasedCalendar);
       });
       it('works for Date.from(iso)', () => {
         const d = Temporal.Date.from(iso);
-        equal(`${d}`, '1970-01-01[c=zerobased]');
+        equal(`${d}`, '1970-01-01[c=zero-based]');
       });
       it('works for Date.from(props)', () => {
-        const d = Temporal.Date.from({ year: 1970, month: 0, day: 1, calendar: 'zerobased' });
-        equal(`${d}`, '1970-01-01[c=zerobased]');
+        const d = Temporal.Date.from({ year: 1970, month: 0, day: 1, calendar: 'zero-based' });
+        equal(`${d}`, '1970-01-01[c=zero-based]');
       });
       it('works for Date.with', () => {
         const d1 = Temporal.Date.from('1970-02-01');
-        const d2 = d1.with({ month: 0, calendar: 'zerobased' });
-        equal(`${d2}`, '1970-01-01[c=zerobased]');
+        const d2 = d1.with({ month: 0, calendar: 'zero-based' });
+        equal(`${d2}`, '1970-01-01[c=zero-based]');
       });
       it('works for Date.withCalendar', () => {
         const d = Temporal.Date.from('1970-01-01');
-        assert(d.withCalendar('zerobased').equals(Temporal.Date.from(iso)));
+        assert(d.withCalendar('zero-based').equals(Temporal.Date.from(iso)));
       });
       it('works for DateTime.from(iso)', () => {
         const dt = Temporal.DateTime.from(iso);
-        equal(`${dt}`, '1970-01-01T00:00[c=zerobased]');
+        equal(`${dt}`, '1970-01-01T00:00[c=zero-based]');
       });
       it('works for DateTime.from(props)', () => {
-        const dt = Temporal.DateTime.from({ year: 1970, month: 0, day: 1, hour: 12, calendar: 'zerobased' });
-        equal(`${dt}`, '1970-01-01T12:00[c=zerobased]');
+        const dt = Temporal.DateTime.from({ year: 1970, month: 0, day: 1, hour: 12, calendar: 'zero-based' });
+        equal(`${dt}`, '1970-01-01T12:00[c=zero-based]');
       });
       it('works for DateTime.with', () => {
         const dt1 = Temporal.DateTime.from('1970-02-01T12:00');
-        const dt2 = dt1.with({ month: 0, calendar: 'zerobased' });
-        equal(`${dt2}`, '1970-01-01T12:00[c=zerobased]');
+        const dt2 = dt1.with({ month: 0, calendar: 'zero-based' });
+        equal(`${dt2}`, '1970-01-01T12:00[c=zero-based]');
       });
       it('works for DateTime.withCalendar', () => {
         const dt = Temporal.DateTime.from('1970-01-01T00:00');
-        assert(dt.withCalendar('zerobased').equals(Temporal.DateTime.from(iso)));
+        assert(dt.withCalendar('zero-based').equals(Temporal.DateTime.from(iso)));
       });
       it('works for YearMonth.from(iso)', () => {
         const ym = Temporal.YearMonth.from(iso);
-        equal(`${ym}`, '1970-01-01[c=zerobased]');
+        equal(`${ym}`, '1970-01-01[c=zero-based]');
       });
       it('works for YearMonth.from(props)', () => {
-        const ym = Temporal.YearMonth.from({ year: 1970, month: 0, calendar: 'zerobased' });
-        equal(`${ym}`, '1970-01-01[c=zerobased]');
+        const ym = Temporal.YearMonth.from({ year: 1970, month: 0, calendar: 'zero-based' });
+        equal(`${ym}`, '1970-01-01[c=zero-based]');
       });
       it('works for MonthDay.from(iso)', () => {
         const md = Temporal.MonthDay.from(iso);
-        equal(`${md}`, '1970-01-01[c=zerobased]');
+        equal(`${md}`, '1970-01-01[c=zero-based]');
       });
       it('works for MonthDay.from(props)', () => {
-        const md = Temporal.MonthDay.from({ month: 0, day: 1, calendar: 'zerobased' });
-        equal(`${md}`, '1972-01-01[c=zerobased]');
+        const md = Temporal.MonthDay.from({ month: 0, day: 1, calendar: 'zero-based' });
+        equal(`${md}`, '1972-01-01[c=zero-based]');
       });
       it('works for TimeZone.getDateTimeFor', () => {
         const tz = Temporal.TimeZone.from('UTC');
         const inst = Temporal.Instant.fromEpochSeconds(0);
-        const dt = tz.getDateTimeFor(inst, 'zerobased');
-        equal(dt.calendar.id, 'zerobased');
+        const dt = tz.getDateTimeFor(inst, 'zero-based');
+        equal(dt.calendar.id, 'zero-based');
       });
       it('works for Instant.toDateTime', () => {
         const inst = Temporal.Instant.fromEpochSeconds(0);
-        const dt = inst.toDateTime('UTC', 'zerobased');
-        equal(dt.calendar.id, 'zerobased');
+        const dt = inst.toDateTime('UTC', 'zero-based');
+        equal(dt.calendar.id, 'zero-based');
       });
       it('works for Temporal.now.dateTime', () => {
-        const nowDateTime = Temporal.now.dateTime('zerobased', 'UTC');
-        equal(nowDateTime.calendar.id, 'zerobased');
+        const nowDateTime = Temporal.now.dateTime('zero-based', 'UTC');
+        equal(nowDateTime.calendar.id, 'zero-based');
       });
       it('works for Temporal.now.date', () => {
-        const nowDate = Temporal.now.date('zerobased', 'UTC');
-        equal(nowDate.calendar.id, 'zerobased');
+        const nowDate = Temporal.now.date('zero-based', 'UTC');
+        equal(nowDate.calendar.id, 'zero-based');
       });
       after(() => {
         Temporal.Calendar.from = originalTemporalCalendarFrom;
diff --git a/spec/abstractops.html b/spec/abstractops.html
index 4c6064e230..8de3db148e 100644
--- a/spec/abstractops.html
+++ b/spec/abstractops.html
@@ -100,13 +100,14 @@ <h1>ToTemporalOverflow ( _normalizedOptions_ )</h1>
     </emu-alg>
   </emu-clause>
 
-  <emu-clause id="sec-temporal-isvalidcalendarname" aoid="IsValidCalendarName">
-    <h1>IsValidCalendarName ( _calendar_ )</h1>
+  <emu-clause id="sec-temporal-isvalidcalendarid" aoid="IsValidCalendarID">
+    <h1>IsValidCalendarID ( _id_ )</h1>
     <p>
-      The IsValidCalendarName abstract operation verifies that the _calendar_ argument (which must be a String value) represents a valid Unicode Calendar Identifier value, according to BCP47.
+      The IsValidCalendarID abstract operation verifies that the _id_ argument (which must be a String value) represents a valid Unicode Calendar Identifier value, according to BCP47.
     </p>
     <p>
-      The abstract operation returns *true* if _calendar_, converted to upper case as described in <emu-xref href="#sec-case-sensitivity-and-case-mapping"></emu-xref>, is equal to one of the Unicode Calendar Identifier values, according to BCP47, converted to uppercase as described in <emu-xref href="#sec-case-sensitivity-and-case-mapping"></emu-xref>. It returns *false* otherwise.
+      The abstract operation returns *true* if _id_ satisfies the syntax of the |CalendarName| production in <emu-xref href="#sec-temporal-iso8601grammar"></emu-xref>.
+      It returns *false* otherwise.
     </p>
   </emu-clause>
 
@@ -465,6 +466,12 @@ <h1>ISO 8601 grammar</h1>
     </ul>
 
     <emu-grammar type="definition">
+      Alpha : one of
+          `A` `B` `C` `D` `E` `F` `G` `H` `I` `J` `K` `L` `M`
+          `N` `O` `P` `Q` `R` `S` `T` `U` `V` `W` `X` `Y` `Z`
+          `a` `b` `c` `d` `e` `f` `g` `h` `i` `j` `k` `l` `m`
+          `n` `o` `p` `q` `r` `s` `t` `u` `v` `w` `x` `y` `z`
+
       Digit : one of
           `0` `1` `2` `3` `4` `5` `6` `7` `8` `9`
 
@@ -612,8 +619,16 @@ <h1>ISO 8601 grammar</h1>
           TimeZoneUTCOffset
           TimeZoneUTCOffset `[` TimeZoneIANAName `]`
 
+      CalChar :
+          Alpha
+          Digit
+
+      CalendarNameComponent :
+          CalChar CalChar CalChar? CalChar? CalChar? CalChar? CalChar? CalChar?
+
       CalendarName :
-          &gt; any string for which IsValidCalendarName returns true
+          CalendarNameComponent
+          CalendarNameComponent `-` CalendarName
 
       Calendar :
           `[c=` CalendarName `]`
diff --git a/spec/calendar.html b/spec/calendar.html
index bf56a724bc..07510e064e 100644
--- a/spec/calendar.html
+++ b/spec/calendar.html
@@ -95,6 +95,8 @@ <h1>Temporal.Calendar ( _id_ )</h1>
       <emu-alg>
         1. If NewTarget is *undefined*, then
           1. Throw a *TypeError* exception.
+        1. If ! IsValidCalendarID(_id_) is *false*, then
+          1. Throw a *RangeError* exception.
         1. Let _calendar_ be ? OrdinaryCreateFromConstructor(NewTarget, *"%Temporal.Calendar.prototype%"*, « [[InitializedTemporalCalendar]], [[Identifier]] »).
         1. Set _calendar_.[[Identifier]] to _id_.
         1. Return _calendar_.