Dates and Days

Days from today

What date is a given span from today

Days and weeks are unambiguous. Months and years are not — one month after 31 January has three defensible answers — so this asks which convention you want instead of picking one silently.

Result— Day of week— Calendar days— Business days— Convention—
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Working — the span, broken down
UnitCount

Why months need a convention and days do not

Ninety days from a date is a single, uncontested answer: count ninety days. One month from 31 January is not. February has twenty-eight days, so the 31st does not exist there, and three different answers are in real use. Clamping gives 28 February, which is what almost every spreadsheet and date library does — silently. Overflowing rolls the three surplus days into March and gives 3 March, 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.

None of these is wrong. What is wrong is applying one without saying so, which is why the answer above always names the convention it used. Days and weeks bypass the question entirely, so if you can express your span in days, do.

Why the answer will not drift when the clocks change

A date here is a year, a month and a day rather than a moment in time. 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. It is the single most common way a date calculator is quietly wrong, and it is invisible except on two days a year.

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 that is the number you actually want, the business days calculator works in it directly and lists every day it removed.

Questions

What is 90 days from today?
Put today's date in the box and ninety in the second, and the answer is above with the weekday and the business-day count beside it. 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 it 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 chosen a convention and not told you. Most of them clamp, so 31 January plus a month gives 28 February. This page will do the same if you leave the month-end setting on clamp, but it also offers overflow, which gives 3 March, and strict, which refuses the calculation as impossible. The point is not that one is right; it is that the answer says which one produced it.
Does a negative number work?
Use the direction control rather than a minus sign — it is clearer to read back, and it means the number in the box always matches the number you said out loud. Selecting \u201cago\u201d 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. Ninety days from the 1st is the 91st day of the sequence, not the 90th, because the day you start on is day zero rather than day one. This is the near-universal reading of \u201cN days from X\u201d and it matches how notice periods are written: the day of the triggering event is excluded and counting begins the next day. If you want the other reading, the days-between calculator offers both explicitly.
How far can it go?
Any date from year 1 to year 9999, in either direction. The arithmetic is exact integer day counting rather than anything that accumulates error, so a span of ten thousand days is as reliable as a span of ten. Spans that would run off either end of that range are refused rather than wrapped around.