Function getTzOffsetTransitions

pkgs/core/src/_lib/test/tzOffsetTransitions.ts:91–143

(year: number)

 */
export function getTzOffsetTransitions(year: number) {
  // start at the end of the previous day
  let date = firstTickInLocalDay(new Date(year, 0, 1));
  if (!isValidDate(date)) {
    throw new Error("Invalid Date");
  }
  let baseTzOffset = previousTickTimezoneOffset(date);
  const transitions = [];
  do {
    let tzOffset = date.getTimezoneOffset();
    if (baseTzOffset !== tzOffset) {
      if (tzOffset !== previousTickTimezoneOffset(date)) {
        // Transition is the first tick of a local day.
        transitions.push({
          date: date,
          type: tzOffset < baseTzOffset ? "forward" : "back",
          before: -baseTzOffset,
          after: -tzOffset,
        });
        baseTzOffset = tzOffset;
      } else {
        // transition was not at the start of the day, so it must have happened
        // yesterday. Back up one day and find the minute where it happened.
        let transitionDate = new Date(date.getTime());
        transitionDate.setDate(transitionDate.getDate() - 1);

        // Iterate through each 5 mins of the day until we find a transition.
        // TODO: this could be optimized to search hours then minutes or by or
        // by using a binary search.
        const dayNumber = transitionDate.getDate();
        while (
          isValidDate(transitionDate) &&
          transitionDate.getDate() === dayNumber
        ) {
          tzOffset = transitionDate.getTimezoneOffset();
          if (baseTzOffset !== tzOffset) {
            transitions.push({
              date: transitionDate,
              type: tzOffset < baseTzOffset ? "forward" : "back",
              before: -baseTzOffset,
              after: -tzOffset,
            });
            baseTzOffset = tzOffset;
            break; // assuming only 1 transition per day
          }
          transitionDate = fiveMinutesLater(transitionDate);
        }
      }
    }
    date = oneDayLater(date);
  } while (date.getFullYear() === year);
  return transitions;
}