Add or Subtract From a Date

Adding a month to 31 January does not give 31 February, because there is no such day. Where the overflow goes is a real decision, and different systems make it differently.

About date arithmetic

Adding days is unambiguous. Adding months is not, and it is the source of most date bugs in software. A month after 31 January is 31 February, which does not exist. This rolls the overflow forward to 3 March in a normal year, which is what PHP and most standard libraries do. Adding 30 days instead lands on 2 March. Both are defensible, and they differ.

That difference matters for anything contractual. "Thirty days from invoice" and "a month from invoice" are not the same deadline, and which one applies depends on the wording rather than on arithmetic. The same is true of notice periods and rental terms.

Years behave the same way at one specific point: a year after 29 February 2024 is 1 March 2025 by this reckoning, since 2025 has no 29 February.

The day of the week is shown because it is frequently the real question — a deadline falling on a Saturday is a different deadline in practice. The ISO week number is there for anyone working to a business calendar.

What it works out

• Adding or subtracting days, weeks, months and years
• The day of the week it lands on
• The ISO week number
• Total days from the starting date

The formula

Apply years, then months, then weeks, then days — in that order

Three months after 21 August 2026 is 21 November 2026. Ten days after that is 1 December 2026, a Tuesday — 102 days from where we started.

The order matters when months are involved. Applying months before days can land on a different date from applying days first, because the month step depends on which month you are in. Years, then months, then weeks, then days is the conventional order and the one used here.

The overflow rule is the part worth knowing. A month after 31 January becomes 3 March in a normal year: the calculation asks for 31 February, finds it does not exist, and rolls forward by the excess. A month after 31 March gives 1 May by the same logic.

If you need exactly thirty days rather than a month, use the days field. The two are different by one to three days depending on the month, and for a payment deadline that difference is real.

Months

Calendar months, which vary in length. Overflow rolls forward.

Weeks

Exactly seven days each. No ambiguity.

Week number

The ISO week of the year, for anyone on a business calendar.

A worked example

Questions

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