Date arithmetic is tested on every build against hand-worked cases, including month ends, leap days and two-digit years. How it is checked
Enter a start date and a span of days, weeks, months or years. Days and weeks have a single answer. For months and years the page asks which month-end convention to use, because one month after 31 January 2026 has three defensible answers.
Why months need a convention and days do not
Ninety days from a date has one uncontested answer: count ninety days. February 2026 has twenty-eight days, though, so one month from 31 January 2026 lands on a 31st that does not exist, and three different answers are in real use. Clamping gives 28 February 2026, which is what almost every spreadsheet and date library does without saying so. Overflowing rolls the three surplus days into March and gives 3 March 2026, which is what you want when a period must never be shortened. Refusing is what a careful contract does: it says the date is impossible and makes somebody decide.
All three are legitimate conventions, and the answer above always names the one it used. Days and weeks avoid the question entirely, so if you can express your span in days, do.
Why the answer does not drift when the clocks change
A date here is a year, a month and a day, with no time of day attached. Every calculation converts to midnight UTC, does integer arithmetic on whole days, and converts back. That matters twice a year: local-time arithmetic across a daylight-saving change gains or loses an hour, and an hour is enough to land an answer on the wrong date.
Business days, alongside
The readout also gives the business-day count over the same span, with weekends and federal holidays removed, because a deadline quoted in calendar days often has to be read against a working calendar. If business days are what you need, the business days calculator counts in them directly and lists every day it removed.
Questions
What is 90 days from today?
The page opens with today's date and 90 days already filled in, so the answer above is 90 days from today, with the weekday and the business-day count beside it. For another span, change the number or the unit. Ninety days is the commonest span asked for by a wide margin, largely because it appears in so many contracts, notice periods and returns policies, and because a quarter of a year is a natural unit to think in, even though a quarter is not exactly ninety days.
Why does adding one month to the 31st give a different answer here than in my spreadsheet?
Because your spreadsheet has picked a convention without telling you. Most of them clamp, so 31 January 2026 plus one month gives 28 February 2026. This page does the same with the month-end setting on clamp. It also offers overflow, which gives 3 March 2026, and strict, which refuses the calculation as impossible. Whichever you choose, the answer names it.
Does a negative number work?
No. Choose “ago (earlier)” in the Direction control and enter the number without a minus sign. That reads back more clearly, and the number in the box always matches the number you said out loud. “Ago” gives the date that many days, weeks, months or years earlier, and the month-end convention applies the same way going backwards as forwards.
Is the start date counted?
No. The day you start on is day zero, so ninety days from the 1st lands on the 91st day of the sequence when the 1st is counted as day one. This is the near-universal reading of “N days from X”, and it matches how notice periods are written: the day of the triggering event is excluded and counting begins the next day. For the other reading, the days-between calculator offers both.
How far can it go?
Any date from year 1 to year 9999, in either direction, and up to 20,000 of the chosen unit. The arithmetic counts whole days as integers, so no error accumulates, and a span of ten thousand days is as reliable as a span of ten. A span of months or years that would run off either end of that range is refused.
Dates and Days