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CalculatorAge Calculator
Enter a birth date and it works out your exact age down to the day, your total time lived in weeks, and how long until your next birthday — including the one date every age calculator has to make a judgement call about.
Exact age
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Why "one month later" is not a fixed number of days
A month is not a stable unit. Add one month to 31 January and there is no 31 February to land on, so every date library has to pick a rule. This calculator counts a full month as having elapsed once the day-of-month has been reached again, and borrows from the days column when it has not — the same logic most calendar apps use for age.
Take 31 January to 3 March. That is one full month (31 Jan → 28 Feb, since February has no 31st) plus 3 more days, so the tool reports 1 month, 3 days — not "1 month, 3 days" measured the other way round, and not a flat 31-day month. The borrow always uses the number of days in the month you are borrowing from, which is why the same gap can read as a different number of days depending on which months it crosses.
The 29 February rule this tool uses
Around 1 in 1,461 people are born on 29 February, and every age calculator has to decide what "birthday" means for them in the three years out of four when that date does not exist. There is no universal standard — UK law and most software default to treating it as 28 February in non-leap years, so that is what this tool does. Some systems instead use 1 March. Neither is "more correct"; they simply pick different sides of the missing day.
| Year | Leap year? | Birthday used |
|---|---|---|
| 2025 | No | 28 February |
| 2026 | No | 28 February |
| 2027 | No | 28 February |
| 2028 | Yes | 29 February |
| 2100 | No — divisible by 100, not by 400 | 28 February |
That last row is the one people forget: century years are leap years only when divisible by 400, so 2100 will not have a 29 February even though 2096 and 2104 do. The calculator applies the full Gregorian rule, not just "divisible by 4".
Worked example
Someone born 15 June 1990, measured as of 17 August 2026, is 36 years, 2 months and 2 days old: 2026 − 1990 = 36 years; August is 2 months after June; the 17th is 2 days after the 15th, with nothing to borrow because both day-of-month values line up cleanly. That same span is 13,212 days, or 1,887 weeks and 3 days over.
Questions people ask
Why does the tool show a different age than I expect?
Usually a time zone issue with the date picker, or the "as of" date being left on an old value. The calculator treats both dates as calendar dates with no time component, so the answer should not depend on what time of day you open the page.
Can I calculate age on a future date, or between two dates that are not today?
Yes — change "Calculate age as of" to any date, past or future. That is also how you would check someone's age on a specific event date, such as a policy start date or an exam eligibility cutoff.
Does this account for time zones or leap seconds?
No, and it should not need to. Age is a calendar concept, not a precise time interval, so the tool works entirely in whole calendar days and ignores clock time and leap seconds.
What happens if I enter a birth date after the "as of" date?
The tool shows an error instead of a negative age. If you want the gap between two dates regardless of order, the date calculator handles that directly.