90 Days From Today: Getting Date Math Right
Why 90 days isn't three months, what happens to 31 January plus one month, and how business-day counting quietly differs from calendar counting.
Someone asks what date falls 90 days from today, and the honest answer is that you can’t do it in your head reliably. The arithmetic isn’t hard. The problem is that months are 28, 29, 30 or 31 days long, and your brain rounds all of them to 30 without telling you.
Try it both ways. Ninety days after 1 January 2026 is 1 April. Ninety days after 1 March 2026 is 30 May. Same ninety days, and the second one lands a day short of the tidy three-month answer, because February got in the way.
Days and months are not interchangeable units
Plenty of real deadlines are written in days on purpose. Net-30 invoice terms, the Schengen 90-in-180 rule, a 90-day prescription supply, the 14-day return window on most EU online purchases. Days are used because days don’t argue — everyone counts them the same way.
Months show up where billing cycles do: rent, subscriptions, loan schedules. Both systems work fine on their own. The mess starts when a contract says “within three months” and someone reads it as 90 days, then discovers at the deadline that the two differ by one or two days depending on which quarter of the year they’re in.
31 January plus one month
That date doesn’t exist, so every piece of software has to pick something. Almost all of them picked the same answer: clamp to the last day of the target month, giving 28 February (or the 29th in a leap year). Excel’s EDATE does it, Google Sheets does it, Python’s dateutil does it, and the Date Calculator does it too, so you can check a spreadsheet against it without surprises.
The alternative — rolling over to 3 March — was tried and dropped, because it wrecks monthly billing. A subscription started on 31 January would drift a few days later every month and eventually fall off the end of the year.
While we’re on spreadsheets: Excel still believes 29 February 1900 existed. It didn’t. 1900 was divisible by 100 and not by 400, so it was an ordinary year. The bug was inherited from Lotus 1-2-3 in the 1980s and kept deliberately for compatibility. Any date arithmetic you do in Excel across that boundary is off by one.
The order you apply the parts changes the answer
Start on 30 January 2026 and add one month and one day.
Months first: 30 January becomes 28 February (clamped), then one more day gives 1 March.
Days first: 30 January becomes 31 January, then one month clamps back down to 28 February.
Two answers, one day apart, from the same instruction. Neither is wrong. That’s why the calculator fixes the order and tells you what it is: years, then months, then weeks and days. A stated order is reproducible; an unstated one is a coin flip you didn’t know you were making.
Business days are not five-sevenths of the calendar
Thirty business days from a Monday is six full weeks later, not “about a month.” That gap catches people out constantly in project plans, because 30 and 42 feel like they should be closer than they are.
Public holidays make it worse, and no calculator can fix that for you. German holidays depend on which state you’re in. UK bank holidays differ in Scotland and Northern Ireland. So the business-day count here skips Saturdays and Sundays only, which makes it a ceiling: knock off your own holidays and you have the real number.
One last thing, and it’s the part people actually came for. Nobody asks about 90 days out of curiosity about the date. They want to know the weekday, because a deadline that lands on a Saturday usually means Friday in practice. Pick your start date and check before you promise anything.