About this calculator
Date arithmetic is full of traps: months of different lengths, leap years, deadlines phrased as "90 days from signing". This calculator handles the two questions that cover nearly every real need — how many days separate two dates (contracts, countdowns, project timelines, "how long since…"), and what date falls a given number of days from a starting point (payment terms, notice periods, trial expirations, "in 30 days it will be…").
The days-between mode also counts business days, skipping Saturdays and Sundays — the unit that deadlines in business and law are so often silently measured in. Note that it does not skip public holidays, which differ by country and state; if a deadline is defined in business days in a contract, check whether holidays are excluded in that definition.
How to use it
- Choose the mode: days between, add days, or subtract days.
- For "days between", enter both dates; order does not matter — the note line tells you which is later.
- For add/subtract, enter the starting date and the number of days.
The formula
Internally the calculator converts each date to a day count since a fixed epoch and subtracts or adds — all leap years (including the century rules: 2000 was a leap year, 1900 was not) are handled by the calendar conversion rather than by counting months by hand, which is where manual attempts go wrong.
Worked example
Jan 1, 2026 to Dec 31, 2026 is 364 days — 52 weeks exactly — containing 260 business days by the convention here (261 weekdays including both endpoints; one endpoint here is the start, which business-day spans conventionally exclude). And 90 days after Oct 20, 2026 is Monday, Jan 18, 2027 — the kind of date a "90-day" contract clause actually lands on.
Frequently asked questions
Does "days between" include both endpoints?
No — it counts the difference, the standard convention. Jan 1 to Jan 2 is 1 day. This matches how deadlines, ages and countdowns are normally defined: a 30-day period starting Jan 1 ends on Jan 31. If you need an inclusive count (for example, hotel nights vs days), add 1.
What exactly is a business day here?
Any Monday through Friday. The count runs from the day after the start date through the end date inclusive — the usual contract convention. Public holidays are not excluded, since they vary by jurisdiction; if your deadline excludes them too, subtract the holidays that fall on weekdays within the span.
How are leap years handled?
Fully, including the Gregorian century rule: years divisible by 4 are leap years, except century years, which are leap only if divisible by 400. So Feb 29 existed in 2000 and 2024, but not in 1900 or 2100. Any span crossing a Feb 29 counts it as a real day.
Why do I get a different answer counting months myself?
Because months are irregular (28–31 days), "three months" is anywhere from 89 to 92 days. Exact date math counts days; approximate month math drifts. When a document says days, count days here; when it says months, the answer is the same date-of-month a set number of months later, which is a different question our Age Calculator's month logic illustrates.