Function getThursdayThisIsoWeek

packages/common/src/i18n/format_date.ts:520–532

(datetime: Date)

 *  ISO Week starts on day 1 (Monday) and ends with day 0 (Sunday)
 */
export function getThursdayThisIsoWeek(datetime: Date) {
  // getDay returns 0-6 range with sunday as 0.
  const currentDay = datetime.getDay();

  // On a Sunday, read the previous Thursday since ISO weeks start on Monday.
  const deltaToThursday = currentDay === 0 ? -3 : THURSDAY - currentDay;

  return createDate(
    datetime.getFullYear(),
    datetime.getMonth(),
    datetime.getDate() + deltaToThursday,
  );
}

function weekGetter(size: number, monthBased = false): DateFormatter {
  return function (date: Date, locale: string) {