Polyfill: 0-based month in ISODateToEpochDays

While investigating #2985 I noticed that ISODateToEpochDays was defined
differently between the spec text and the reference code. Use 0-based
months as in the spec text, to avoid confusion when comparing the two.

Also I noticed that it can now be implemented more efficiently with
GetUTCEpochMilliseconds.

polyfill/lib/ecmascript.mjs

@@ -2858,8 +2858,8 @@ export function UnbalanceDateDurationRelative(dateDuration, plainRelativeTo) {
   // balance years, months, and weeks down to days
   const isoDate = TemporalObjectToISODateRecord(plainRelativeTo);
   const later = CalendarDateAdd(GetSlot(plainRelativeTo, CALENDAR), isoDate, yearsMonthsWeeksDuration, 'constrain');
-  const epochDaysEarlier = ISODateToEpochDays(isoDate.year, isoDate.month, isoDate.day);
-  const epochDaysLater = ISODateToEpochDays(later.year, later.month, later.day);
+  const epochDaysEarlier = ISODateToEpochDays(isoDate.year, isoDate.month - 1, isoDate.day);
+  const epochDaysLater = ISODateToEpochDays(later.year, later.month - 1, later.day);
   const yearsMonthsWeeksInDays = epochDaysLater - epochDaysEarlier;
   return dateDuration.days + yearsMonthsWeeksInDays;
 }
@@ -3142,10 +3142,9 @@ function CombineDateAndNormalizedTimeDuration(dateDuration, norm) {
   return { date: dateDuration, norm };
 }
 
+// Caution: month is 0-based
 function ISODateToEpochDays(y, m, d) {
-  // This is inefficient, but we use GetUTCEpochNanoseconds to avoid duplicating
-  // the workarounds for legacy Date. (see that function for explanation)
-  return GetUTCEpochNanoseconds(y, m, d, 0, 0, 0, 0, 0, 0).divide(DAY_NANOS).toJSNumber();
+  return GetUTCEpochMilliseconds(y, m + 1, d, 0, 0, 0, 0) / DAY_MS;
 }
 
 export function DifferenceISODate(y1, m1, d1, y2, m2, d2, largestUnit = 'days') {
@@ -3185,7 +3184,8 @@ export function DifferenceISODate(y1, m1, d1, y2, m2, d2, largestUnit = 'days')
   const constrained = ConstrainISODate(intermediate.year, intermediate.month, d1);
 
   let weeks = 0;
-  let days = ISODateToEpochDays(y2, m2, d2) - ISODateToEpochDays(constrained.year, constrained.month, constrained.day);
+  let days =
+    ISODateToEpochDays(y2, m2 - 1, d2) - ISODateToEpochDays(constrained.year, constrained.month - 1, constrained.day);
 
   if (largestUnit === 'week') {
     weeks = MathTrunc(days / 7);