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Date Calculator

“90 days from today”, “18 months before the deadline” — land on the exact date, weekday included, with the month-arithmetic quirks handled the way calendars actually handle them.

Shift a date ± years · months · weeks · days

Resulting date

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Calendar distance

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total days between the two dates

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§1How the arithmetic works

order of operations: years → months → weeks & days months first shift the month number; if the day doesn't exist in the target month, it clamps to the month's last day: 31 Jan + 1 month = 28 Feb (29 in a leap year) weeks & days are then exact 7-day / 1-day steps — never approximate.

This is the convention contracts, banks and spreadsheets follow. It also means month arithmetic is not reversible: 28 Feb − 1 month = 28 Jan, not 31 Jan.

Worked example — a 90-day visa run from 31 July 2026

31 Jul + 90 days: Aug has 31 → 59 left after 31 Aug Sep has 30 → 29 left after 30 Sep 29 Oct 2026 (a Thursday) compare: 31 Jul + 3 months = 31 Oct — two days later than “90 days”.

Every “90 days ≈ 3 months” assumption drifts by 0–3 days depending on which months it crosses. For anything with a penalty attached, compute the exact form the rule uses.

§2The gotchas

“A month” is not 30 days

“3 months from 15 Nov” is 15 Feb (92 days), while “90 days from 15 Nov” is 13 Feb. Notice periods, visas and warranties differ on which they mean — check the wording before you count.

Deadlines count from the day after

Most legal “within N days” clauses exclude the trigger day itself and, if the deadline lands on a holiday, roll to the next working day. This page gives the raw calendar answer; apply the clause's own rules on top.

§3Related

Day counter for the gap between two known dates, day of the week for any single date.