Split non-ZonedDateTime code path out of NanosecondsToDays

Similar to the previous commit, this makes NanosecondsToDays easier to
understand and reason about. After splitting BalanceTimeDuration and
BalanceTimeDurationRelative, there was only one remaining call site of
NanosecondsToDays with a possible non-ZonedDateTime relativeTo, in
RoundDuration. Instead, inline the 24-hour day case into the call site.

UPSTREAM_COMMIT=dffb9298e0c95c8212a624058652ee7f8dba2cdd

lib/ecmascript.ts

@@ -3735,24 +3735,18 @@ export function TotalDurationNanoseconds(
   return JSBI.add(JSBI.BigInt(nanoseconds), JSBI.multiply(microseconds, THOUSAND));
 }
 
-function NanosecondsToDays(nanosecondsParam: JSBI, relativeTo: ReturnType<typeof ToRelativeTemporalObject>) {
+function NanosecondsToDays(nanosecondsParam: JSBI, zonedRelativeTo: Temporal.ZonedDateTime) {
   const TemporalInstant = GetIntrinsic('%Temporal.Instant%');
   const sign = MathSign(JSBI.toNumber(nanosecondsParam));
   let nanoseconds = JSBI.BigInt(nanosecondsParam);
-  let dayLengthNs = 86400e9;
-  if (sign === 0) return { days: 0, nanoseconds: ZERO, dayLengthNs };
-  if (!IsTemporalZonedDateTime(relativeTo)) {
-    let days: JSBI;
-    ({ quotient: days, remainder: nanoseconds } = divmod(nanoseconds, JSBI.BigInt(dayLengthNs)));
-    return { days: JSBI.toNumber(days), nanoseconds, dayLengthNs };
-  }
+  if (sign === 0) return { days: 0, nanoseconds: ZERO, dayLengthNs: JSBI.toNumber(DAY_NANOS) };
 
-  const startNs = GetSlot(relativeTo, EPOCHNANOSECONDS);
-  const start = GetSlot(relativeTo, INSTANT);
+  const startNs = GetSlot(zonedRelativeTo, EPOCHNANOSECONDS);
+  const start = GetSlot(zonedRelativeTo, INSTANT);
   const endNs = JSBI.add(startNs, nanoseconds);
   const end = new TemporalInstant(endNs);
-  const timeZone = GetSlot(relativeTo, TIME_ZONE);
-  const calendar = GetSlot(relativeTo, CALENDAR);
+  const timeZone = GetSlot(zonedRelativeTo, TIME_ZONE);
+  const calendar = GetSlot(zonedRelativeTo, CALENDAR);
 
   // Find the difference in days only.
   const dtStart = GetPlainDateTimeFor(timeZone, start, calendar);
@@ -3817,6 +3811,7 @@ function NanosecondsToDays(nanosecondsParam: JSBI, relativeTo: ReturnType<typeof
 
   let isOverflow = false;
   let relativeInstant = new TemporalInstant(intermediateNs);
+  let dayLengthNs;
   do {
     // calculate length of the next day (day that contains the time remainder)
     const oneDayFartherNs = AddZonedDateTime(relativeInstant, timeZone, calendar, 0, 0, 0, sign, 0, 0, 0, 0, 0, 0);
@@ -6193,15 +6188,18 @@ export function RoundDuration(
   // larger. We'll cast away `undefined` when it's used lower down below.
   let dayLengthNs: JSBI | undefined;
   if (unit === 'year' || unit === 'month' || unit === 'week' || unit === 'day') {
-    nanoseconds = TotalDurationNanoseconds(0, hours, minutes, seconds, milliseconds, microseconds, nanosecondsParam, 0);
-    let intermediate;
+    nanoseconds = TotalDurationNanoseconds(0, hours, minutes, seconds, milliseconds, microseconds, nanoseconds, 0);
+    let deltaDays;
     if (zdtRelative) {
-      intermediate = MoveRelativeZonedDateTime(zdtRelative, years, months, weeks, days);
+      const intermediate = MoveRelativeZonedDateTime(zdtRelative, years, months, weeks, days);
+      let dayLengthNumber;
+      ({ days: deltaDays, nanoseconds, dayLengthNs: dayLengthNumber } = NanosecondsToDays(nanoseconds, intermediate));
+      dayLengthNs = JSBI.BigInt(dayLengthNumber);
+    } else {
+      ({ quotient: deltaDays, remainder: nanoseconds } = divmod(nanoseconds, DAY_NANOS));
+      deltaDays = JSBI.toNumber(deltaDays);
+      dayLengthNs = DAY_NANOS;
     }
-    let deltaDays;
-    let dayLength: number;
-    ({ days: deltaDays, nanoseconds, dayLengthNs: dayLength } = NanosecondsToDays(nanoseconds, intermediate));
-    dayLengthNs = JSBI.BigInt(dayLength);
     days += deltaDays;
     hours = minutes = seconds = milliseconds = microseconds = 0;
   }