Date Calculator
Count the days between two dates, add or subtract any period from a date, and get an exact age in years, months and days — with working days broken out separately.
Why calendar arithmetic is not just division
Months have different lengths, so "three months from 31 January" has no obvious answer — 31 April does not exist. The convention used here, and by most software, is to clamp to the last valid day of the target month, giving 30 April. Worth knowing, because it means adding one month and then subtracting one month does not always return you to where you started.
Years have the same problem in miniature. Adding one year to 29 February 2024 gives 28 February 2025, and adding four gives 29 February 2028.
Leap year rule, in full
- Divisible by 4 — leap year
- Unless divisible by 100 — not a leap year
- Unless also divisible by 400 — leap year after all
So 1900 was not a leap year and 2000 was. This is the rule that broke a surprising amount of software in early 2000, and the reason a few systems still disagree about dates in February.
Frequently asked questions
Is the end date counted in the total?
By default no — the difference between 1 March and 3 March is two days, which is how date arithmetic works everywhere from spreadsheets to contracts. Switch on "include both dates" when you are counting occupancy or billing days, where a stay from the 1st to the 3rd is three days.
How is exact age calculated?
By counting whole years first, then whole months from that anniversary, then the remaining days — not by dividing total days by 365.25. Averaging gets birthdays wrong near leap years and near month boundaries, which is precisely when people check.
What counts as a working day?
Monday to Friday, excluding weekends. Public holidays are not included because they differ by country, region and sometimes by employer. For a legally meaningful deadline, take the working-day figure here and subtract the holidays that apply to your jurisdiction.
Does this handle leap years and time zones?
Leap years yes — February 29 is handled correctly, including the rule that century years are only leap years when divisible by 400. Time zones are not modelled: everything is treated as a plain calendar date, which is what you want for birthdays, deadlines and durations, and is not what you want for anything involving a specific instant in time.
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