Function getFirstThursdayOfYear

packages/common/src/i18n/format_date.ts:509–516

(year: number)

const JANUARY = 0;
const THURSDAY = 4;
function getFirstThursdayOfYear(year: number) {
  const firstDayOfYear = createDate(year, JANUARY, 1).getDay();
  return createDate(
    year,
    0,
    1 + (firstDayOfYear <= THURSDAY ? THURSDAY : THURSDAY + 7) - firstDayOfYear,
  );
}

/**
 *  ISO Week starts on day 1 (Monday) and ends with day 0 (Sunday)