Date Calculator
Calculate the number of days, weeks, months and years between two dates, or add/subtract a duration from a date.
About the Date Calculator
"How many days until" and "what date is 90 days from now" are two versions of the same underlying problem, and this calculator handles both. The difference mode answers questions like how many days are left until a deadline, how long a trip lasted, or how many days someone has been employed; the add/subtract mode answers the reverse — given a starting date, what date do you land on after adding or subtracting a stretch of days, weeks, months or years.
It shows up in contract and legal work more than people expect — a 30-day notice period, a 90-day payment term, a 180-day residency test — where the exact date matters and "about three months" isn't good enough. It's just as useful for ordinary planning: counting down to a trip, working out a project deadline, or figuring out what week a due date falls in.
Because month lengths aren't uniform, adding "3 months" or counting the days between two dates six months apart isn't simple multiplication — this handles the calendar's irregularities (28/29/30/31-day months, leap years) so the result matches what a calendar would actually show.
How it’s calculated
In difference mode, the calculator takes a straight day count between the two dates and separately works out a years/months/days breakdown by counting whole calendar units, the same way you'd count by hand — so "14 months" and "1 year, 2 months" both appear, letting you use whichever framing fits.
In add/subtract mode, adding months or years moves the calendar field directly (so adding 1 month to January 31 rolls forward using JavaScript's date arithmetic, which normalizes overflow into the next month) rather than adding a fixed number of days, which is what keeps "add 1 month" behaving the way a calendar would, not like adding exactly 30 days.
Frequently asked questions
How many days are there between two dates?
Enter both dates in difference mode and the calculator gives you the total day count along with a years/months/days breakdown, so you can see the span both as a single number and as a calendar-style duration.
Does the day count include both the start and end date?
The total is the number of days between the two dates, not an inclusive count of both endpoints — if you need an inclusive count (common for things like "how many days do I have off," counting both the first and last day), add 1 to the result.
What date is 90 days from today?
Switch to add/subtract mode, set the amount to 90, the unit to days, and the direction to add, starting from today's date — the calculator returns the exact resulting calendar date.
How does adding months handle different month lengths?
It moves forward by calendar months rather than a fixed day count, so adding a month to a date in a 31-day month and one in a 30-day month still land on the corresponding day the following month wherever that's possible.
Does this calculator account for leap years?
Yes — because all the arithmetic operates on real calendar dates rather than a fixed 365-day year, leap years are already reflected in both the difference and add/subtract results.
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