Date Calculator

What is 31 January plus one month?

Add one month to 31 January 2024 and this calculator returns 29 February 2024, a Thursday. Not 2 March, and not 28 February — because 2024 is a leap year.

That single example contains the whole difficulty of date arithmetic. There is no 31 February, so "plus one month" has no literal answer and a convention must be chosen. The near-universal convention, and the one used here, is to clamp to the last valid day of the target month. Add one month to 31 January and you land on the final day of February, whichever day that is.

Adding days has no such ambiguity — every day is followed by exactly one other day. Adding months and years does, because those units have variable length.

How each unit behaves

Days and weeks — pure counting. Every day advances the calendar by one, leap days included. A week is seven days, always.

Months — take the same day number in the target month, and clamp down if it does not exist. 31 March minus one month gives 28 or 29 February. 31 August plus one month gives 30 September.

Years — take the same date in the target year, with one exception: 29 February plus one year clamps to 28 February, since the target year has no 29th.

The clamping rule has a consequence worth knowing: the operation is not reversible. 31 January plus one month is 29 February, but 29 February minus one month is 29 January, not 31 January. Information is lost at the clamp, so adding and then subtracting the same interval does not always return you to the start.

Nor is it associative. Adding one month twice to 31 January gives 29 February, then 29 March. Adding two months at once gives 31 March. Both are defensible; they are simply different operations.

When this matters

Contract and notice dates. A three-month notice period served from 30 November ends on the last day of February, not in March.

Subscription and billing cycles. A subscription started on the 31st bills on the 28th, 29th or 30th in shorter months, and most billing systems return to the 31st afterwards rather than staying on the clamped day.

Warranty and guarantee expiry. A two-year warranty from 29 February 2024 expires on 28 February 2026.

Statutory deadlines. Tax, compliance and filing windows are usually expressed in months from a trigger date, and the clamping convention decides the final day.

Age and eligibility cutoffs, where a single day either side changes the answer.

Reading the result

The day of the week is often the operationally important part. A deadline that lands on a Sunday or a public holiday usually moves, and most legal and banking conventions push it to the next working day — though some push backwards instead, so the governing rule matters.

Note that this calculator works in calendar days, not business days. A 90-day period includes weekends and holidays. If your requirement is 90 *working* days, the calendar date will be roughly four and a half months out rather than three.

Common mistakes

Assuming a month is 30 days. Across a year the error accumulates to about five days.

Expecting add and subtract to cancel. Clamping makes the operation lossy.

Treating calendar days as business days. Courts, banks and payroll usually mean one specific one of the two.

Ignoring the leap-year rule. A year divisible by 4 is a leap year, except centuries, unless divisible by 400 — so 1900 was not and 2000 was.

Reading DD/MM and MM/DD ambiguously. 05/06/2025 is two different dates depending on convention, which is why this calculator uses an explicit date picker.