Formula
Result = start date ± (years and months on the calendar) ± (7 × weeks + days) How it works
Years and months are applied first, on the calendar: adding one month to March 15 gives April 15, whatever the length of March. Weeks and days are then added as exact 24-hour days.
If the target month is too short for the starting day — for example January 31 plus one month — the result is moved back to the last day of that month (February 28, or 29 in a leap year). This matches how most banks, contracts and spreadsheet functions such as EDATE behave.
Typical uses include working out a 90-day return window, a notice period, a contract renewal, the end of a probation period or the date a certain number of weeks ago.
Frequently asked questions
Is the start date counted as day 0 or day 1?
Day 0. Adding 1 day to Monday gives Tuesday. If your rule says “within 10 days including today”, add 9.
Why not just treat a month as 30 days?
Because calendar months range from 28 to 31 days. Use the Days box with 30 if your rule really means 30 days rather than one month.