Is this week odd or even?

A continuous fortnightly rota needs an anchor

Choose a specific Monday as the start of week A. 28 December 2026 is odd week 53 and 4 January 2027 is odd week 1, but a continuous A/B rota must still alternate. The recipe counts whole calendar weeks relative to the anchor, including dates before it.

Use isoWeekDate from the API recipe first. alternatingWeek("2027-01-04", "2026-12-28") returns B. With date A2 and Monday anchor H2, =MOD(INT((A2-$H$2)/7),2) returns 0 for A and 1 for B.

function alternatingWeek(value, anchor) {
  isoWeekDate(value);
  if (isoWeekDate(anchor).isoWeekday !== 1) throw new RangeError("Anchor must be a Monday");
  const days = (Date.parse(value + "T00:00:00Z") - Date.parse(anchor + "T00:00:00Z")) / 86400000;
  const block = Math.floor(days / 7);
  return ((block % 2) + 2) % 2 === 0 ? "A" : "B";
}