WIP examples that can't be done yet

docs/cookbook/customTimeZone.js

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+function formatOffsetString(sign, offset) {
+  const hours = `${offset.hours}`.padStart(2, '0');
+  const minutes = `${offset.minutes}`.padStart(2, '0');
+  return `${sign < 0 ? '-' : '+'}${hours}:${minutes}`;
+}
+
+const nsPerDay = 86400_000_000_000n;
+const nsPerMillisecond = 1_000_000n;
+
+function Day(t) {
+  return Math.floor(t / nsPerDay);
+}
+
+function MakeDate(day, time) {
+  return day * nsPerDay + time;
+}
+
+function MakeDay(year, month, day) {
+  const m = month - 1;
+  const ym = year + Math.floor(m / 12);
+  const mn = m % 12;
+  const t = BigInt(Date.UTC(ym, mn, 1)) * nsPerMillisecond;
+  return Day(t) + day - 1;
+}
+
+function MakeTime(h, min, s, ms, µs, ns) {
+  const MinutesPerHour = 60n;
+  const SecondsPerMinute = 60n;
+  const nsPerSecond = 1_000_000_000n;
+  const nsPerMinute = nsPerSecond * SecondsPerMinute;
+  const nsPerHour = nsPerMinute * MinutesPerHour;
+  return (
+    BigInt(h) * nsPerHour +
+    BigInt(min) * nsPerMinute +
+    BigInt(s) * nsPerSecond +
+    BigInt(ms) * nsPerMillisecond +
+    BigInt(µs) * 1000n +
+    BigInt(ns)
+  );
+}
+
+class OffsetTimeZone extends Temporal.TimeZone {
+  constructor(sign = 1, hours = 0, minutes = 0, seconds = 0, milliseconds = 0, microseconds = 0, nanoseconds = 0) {
+    const hourString = `${hours}`.padStart(2, '0');
+    const minuteString = `${minutes}`.padStart(2, '0');
+    const name = `${sign < 0 ? '-' : '+'}${hourString}:${minuteString}`;
+    super(name);
+    this._offsetNs = BigInt(sign) * MakeTime(hours, minutes, seconds, milliseconds, microseconds, nanoseconds);
+  }
+
+  // Implementations of TimeZone protocol //////////////////////////////////////
+
+  getOffsetAtInstant(/* epochNs */) {
+    return this._offsetNs; // offset is always the same
+  }
+
+  possibleInstants(isoYear, isoMonth, isoDay, hour, minute, second, millisecond, microsecond, nanosecond) {
+    const epochNs = MakeDate(
+      MakeDay(isoYear, isoMonth, isoDay),
+      MakeTime(hour, minute, second, millisecond, microsecond, nanosecond)
+    );
+    return [epochNs + this._offsetNs];
+  }
+
+  // (optional?) Implementations of public API /////////////////////////////////
+
+  // eslint-disable-next-line require-yield
+  *getTransitions() {
+    return null; // no transitions ever
+  }
+
+  // Public API that should be inherited from TimeZone /////////////////////////
+
+  getOffsetFor(absolute) {
+    return formatOffsetString(this.getOffsetAtInstant(absolute));
+  }
+
+  // Make ES.GetTimeZoneDateTimeParts take an offset instead of a time zone id
+  /*getDateTimeFor(absolute) {
+    const ns = absolute.getEpochNanoseconds();
+    const {
+      year,
+      month,
+      day,
+      hour,
+      minute,
+      second,
+      millisecond,
+      microsecond,
+      nanosecond
+    } = ES.GetTimeZoneDateTimeParts(ns, this.getOffsetAtInstant(ns));
+    return new Temporal.DateTime(year, month, day, hour, minute, second, millisecond, microsecond, nanosecond);
+  }*/
+
+  getAbsoluteFor(dateTime, options = undefined) {
+    options ??= { disambiguation: 'earlier' };
+    const { disambiguation } = options;
+    const { year, month, day, hour, minute, second, millisecond, microsecond, nanosecond } = dateTime;
+    const possibleInstants = this.possibleInstants(
+      year,
+      month,
+      day,
+      hour,
+      minute,
+      second,
+      millisecond,
+      microsecond,
+      nanosecond
+    );
+    if (possibleInstants.length === 1) return new Temporal.Absolute(possibleInstants[0]);
+    if (possibleInstants.length) {
+      switch (disambiguation) {
+        case 'earlier':
+          return new Temporal.Absolute(possibleInstants[0]);
+        case 'later':
+          return new Temporal.Absolute(possibleInstants[1]);
+        case 'reject':
+          throw new RangeError('multiple absolute found');
+      }
+    }
+
+    const utcns = ES.GetEpochFromParts(year, month, day, hour, minute, second, millisecond, microsecond, nanosecond);
+    if (utcns === null) throw new RangeError('DateTime outside of supported range');
+    const dayNs = 86400n * BigInt(1e9);
+    const before = this.getOffsetAtInstant(utcns - dayNs);
+    const after = this.getOffsetAtInstant(utcns + dayNs);
+    const diff = after.minus(before);
+    switch (disambiguation) {
+      case 'earlier': {
+        const earlier = dateTime.minus(diff);
+        return this.getAbsoluteFor(earlier, disambiguation);
+      }
+      case 'later': {
+        const later = dateTime.plus(diff);
+        return this.getAbsoluteFor(later, disambiguation);
+      }
+      case 'reject': {
+        throw new RangeError('no such absolute found');
+      }
+    }
+  }
+}