Adding or subtracting days from a date, by hand
A deadline that's "90 days out," a return window that "closes in 45 days," a lease that started "60 days ago" — these are all the same question: move forward or backward some number of days from a known date, and land on the actual calendar date and weekday. Doing it in your head past a month boundary is where people go wrong, because months don't all have the same length.
The method
- Start from the known date.
- Count forward (add) or backward (subtract) the given number of days, carrying over into the next or previous month whenever you run past its end.
- Read off the resulting date, then work out its day of the week by counting from a date you already know.
In one line: result date = start date ± n days, with the sign matching "from" (forward, add) or "before" (backward, subtract).
A worked example: adding days
A payment term is 45 days from January 15, 2026.
January has 31 days, so from January 15 there are 31 − 15 = 16 days left in January. That leaves 45 − 16 = 29 days to carry into February. February 2026 isn't a leap year (2026 isn’t divisible by 4), so it has 28 days — 29 − 28 = 1 day left over, landing on March 1, 2026, a Sunday.
A worked example: subtracting days
A lease's notice period means counting back 60 days before November 20, 2026.
From November 20 back to November 1 is 19 days, with 60 − 19 = 41 still to go. Stepping back through October (31 days) leaves 41 − 31 = 10 more days into September, landing on September 21, 2026 — a Monday.
The mistake worth watching for
The usual error is treating every month as 30 days, which drifts the answer by a day or two per month crossed — harmless for a casual guess, costly for a legal notice period or a payment due date with a penalty attached. The other trap is leap years: February only gets its 29th day when the year is divisible by 4 (and, for century years, by 400 but not 100) — skip that check and a calculation spanning late February can land a day off.
The free Add / Subtract Days tool does this carry-the-month arithmetic exactly, for any date and any number of days forward or back, and shows the resulting weekday alongside it.