diff --git a/polyfill/lib/calendar.mjs b/polyfill/lib/calendar.mjs
index b2b68efee2..4ac2f2d8e4 100644
--- a/polyfill/lib/calendar.mjs
+++ b/polyfill/lib/calendar.mjs
@@ -633,18 +633,20 @@ const nonIsoHelperBase = {
   adjustCalendarDate(calendarDate, cache, overflow /*, fromLegacyDate = false */) {
     if (this.calendarType === 'lunisolar') throw new RangeError('Override required for lunisolar calendars');
     this.validateCalendarDate(calendarDate);
-    const largestMonth = this.monthsInYear(calendarDate, cache);
-    let { month, year, eraYear, monthCode } = calendarDate;
-
     // For calendars that always use the same era, set it here so that derived
     // calendars won't need to implement this method simply to set the era.
     if (this.constantEra) {
       // year and eraYear always match when there's only one possible era
-      if (year === undefined) year = eraYear;
-      if (eraYear === undefined) eraYear = year;
-      calendarDate = { ...calendarDate, era: this.constantEra, year, eraYear };
+      const { year, eraYear } = calendarDate;
+      calendarDate = {
+        ...calendarDate,
+        era: this.constantEra,
+        year: year !== undefined ? year : eraYear,
+        eraYear: eraYear !== undefined ? eraYear : year
+      };
     }
-
+    const largestMonth = this.monthsInYear(calendarDate, cache);
+    let { month, monthCode } = calendarDate;
     ({ month, monthCode } = resolveNonLunisolarMonth(calendarDate, overflow, largestMonth));
     return { ...calendarDate, month, monthCode };
   },
@@ -1146,10 +1148,10 @@ const helperHebrew = ObjectAssign({}, nonIsoHelperBase, {
       } else {
         if (overflow === 'reject') {
           ES.RejectToRange(month, 1, this.monthsInYear({ year }));
-          ES.RejectToRange(day, 1, this.maximumMonthLength(calendarDate));
+          ES.RejectToRange(day, 1, this.maximumMonthLength({ year, month }));
         } else {
           month = ES.ConstrainToRange(month, 1, this.monthsInYear({ year }));
-          day = ES.ConstrainToRange(day, 1, this.maximumMonthLength({ ...calendarDate, month }));
+          day = ES.ConstrainToRange(day, 1, this.maximumMonthLength({ year, month }));
         }
         if (monthCode === undefined) {
           monthCode = this.getMonthCode(year, month);
@@ -1298,10 +1300,8 @@ const helperIndian = ObjectAssign({}, nonIsoHelperBase, {
  * eras. Note that it mutates and normalizes the original era objects, which is
  * OK because this is non-observable, internal-only metadata.
  *
- * The result is an array of eras with this shape:
- * ```
- * interface Era {
- *   // name of the era
+ *  interface Era {
+ *   /** name of the era
  *   name: string;
  *
  *   // alternate name of the era used in old versions of ICU data
@@ -1309,25 +1309,29 @@ const helperIndian = ObjectAssign({}, nonIsoHelperBase, {
  *   // with the oldest era being 0.
  *   genericName: string;
  *
- *   // Signed calendar year where this era begins. Will be
- *   // 1 (or 0 for zero-based eras) for the anchor era assuming that `year`
- *   // numbering starts at the beginning of the anchor era, which is true
- *   // for all ICU calendars except Japanese. If an era starts mid-year
- *   // then a calendar month and day are included. Otherwise
- *   // `{ month: 1, day: 1 }` is assumed.
- *   anchorEpoch:  { year: number } | { year: number, month: number, day: number }
+ *   // Signed calendar year where this era begins. Will be 1 (or 0 for zero-based
+ *   // eras) for the anchor era assuming that `year` numbering starts at the
+ *   // beginning of the anchor era, which is true for all ICU calendars except
+ *   // Japanese. For input, the month and day are optional. If an era starts
+ *   // mid-year then a calendar month and day are included.
+ *   // Otherwise `{ month: 1, day: 1 }` is assumed.
+ *   anchorEpoch: { year: number; month: number; day: number };
  *
  *   // ISO date of the first day of this era
- *   isoEpoch: { year: number, month: number, day: number}
+ *   isoEpoch: { year: number; month: number; day: number };
  *
  *   // If present, then this era counts years backwards like BC
  *   // and this property points to the forward era. This must be
  *   // the last (oldest) era in the array.
- *   reverseOf: Era;
+ *   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 = false;
+ *   hasYearZero?: boolean;
+ *
+ *   // Override if this era is the anchor. Not normally used because
+ *   // anchor eras are inferred.
+ *   isAnchor?: boolean;
  * }
  * ```
  * */
@@ -1387,6 +1391,7 @@ function adjustEras(eras) {
   ArraySort.call(eras, (e1, e2) => {
     if (e1.reverseOf) return 1;
     if (e2.reverseOf) return -1;
+    if (!e1.isoEpoch || !e2.isoEpoch) throw new RangeError('Invalid era data: missing ISO epoch');
     return e2.isoEpoch.year - e1.isoEpoch.year;
   });