Three calculators in one
Date arithmetic is the kind of thing that looks trivial and then quietly goes wrong. Months have different lengths, leap years interrupt the pattern, and "three months from the 31st" has no obvious answer. The three tabs above cover the calculations people actually need.
Add or subtract time
Start from a date and time, then add or subtract a number of days, hours and minutes. Use it to answer questions like "if I start a 36-hour process on Friday at 2pm, when does it finish?" or "what date is 90 days before the deadline?" The result rolls correctly across midnight, month ends and year boundaries, so you never have to count on your fingers past the end of February.
Date difference
Enter two dates and times to get the exact gap between them. This is the tab for counting days until a deadline, working out how long a project ran, or checking how much notice a contract actually requires. One thing worth being deliberate about: the difference between 1 March and 4 March is three days, but the period "covers" four calendar days if you count both endpoints. Contracts, hotel bookings and notice periods differ on which they mean, so check which convention applies before relying on the number.
Age calculator
Enter a date of birth to get an exact age in years, months and days, along with the total number of days lived. It handles the leap-year edge cases properly, including birthdays on 29 February — a date that only exists in one year out of four, which is why most legal systems treat such a birthday as falling on 28 February or 1 March in ordinary years, depending on the jurisdiction.
Why date arithmetic is harder than it looks
- Months are not a fixed length. Adding "one month" to 31 January has no universally agreed answer — 28 February, 2 March and 3 March are all defensible. This is why the add/subtract tab works in days, hours and minutes, which are unambiguous.
- Leap years are not simply every four years. A year is a leap year if it is divisible by 4, except centuries, unless divisible by 400. So 1900 was not a leap year but 2000 was — a rule that broke a great deal of software written by people who only knew the first half.
- Days are not always 24 hours. In regions observing daylight saving, one day each year is 23 hours long and another is 25. A duration measured in hours across a clock change will not match the same duration measured in days.
- Time zones shift the answer. The same instant falls on different calendar dates depending on where you are standing.
What people calculate
- Deadlines and notice periods — resignation dates, contract expiry, statutory deadlines counted in working or calendar days.
- Project planning — when a task that takes 45 days will actually finish.
- Travel — arrival times after a long flight, visa validity windows, trip lengths.
- Billing and payroll — hours worked between two timestamps, invoice terms such as net 30.
- Personal milestones — exact age for a form, how long you have been at a job, days until a due date, anniversaries in days rather than years.
- Cooking and processes — long brines, ferments, curing times.
Frequently Asked Questions
How is age calculated?
As complete years, then remaining months, then remaining days from the date of birth to today — the way age is stated in everyday use and on official forms. The total number of days lived is shown alongside it.
Which time zone are my entries interpreted in?
Your device's local time zone. If you are calculating across zones, convert to a single zone first — the world clock is useful for that.
Does it account for leap years?
Yes, including the century rule, so results spanning 2000 or 2100 are correct.
Does the difference include both the start and end day?
It reports the elapsed time between the two moments, so 1 March to 4 March is three days. If your context counts both endpoints, add one.
Can I count only working days?
Not currently — the calculation is in calendar days. For planning around public holidays, see the holiday calendar.
Can I subtract as well as add?
Yes, switch the operation to Subtract on the first tab to work backwards from a date.
Is my data sent anywhere?
No. Every calculation runs in your browser; dates of birth and everything else stay on your device.
Related tools
To watch the time remaining tick down rather than calculate it once, use the countdown. For durations of minutes and hours, the timer and stopwatch are more direct.