General
Age Calculator
Works an age, where whole units rather than elapsed time is the convention.
Result
Ages count completed months using real calendar month lengths, which is why the day count can shift depending on which months the period spans.
Get a Free Business AuditHow the calculation works
Age is the number of complete years since a date, with the remaining months and days shown separately. Someone is a given age until the day before their next birthday, which is a counting convention rather than elapsed time and is why age does not round.
What moves the number
- The reference date the age is calculated at.
- Leap years within the period.
- Time zone, where the dates are in different zones.
- Whether partial years should be shown.
Where this one goes wrong
Leap years mean a period in days does not divide evenly into years, which is why age is calculated by comparing calendar dates rather than by dividing elapsed days by a year length, and the latter produces an off-by-one error around birthdays. Compare the dates. The February the twenty-ninth birthday is the known edge case, and different jurisdictions and systems treat the non-leap-year birthday differently.
Before you quote
- Compare calendar dates rather than dividing elapsed days.
- State the reference date the age applies at.
- Be aware of the leap day birthday edge case.
Questions
About this calculator
Why not just divide the days by 365?
Because leap years mean days do not divide evenly into years, and the result produces an off-by-one error around birthdays. Age is calculated by comparing calendar dates, which is why it steps rather than accumulating smoothly.
What happens with a leap day birthday?
It is a genuine edge case, and different jurisdictions and software treat the non-leap-year birthday differently, some using the twenty-eighth of February and some the first of March. It matters for legal thresholds more than for arithmetic.
Does age round up?
No. Someone is a given age until the day before their next birthday, which is a counting convention rather than elapsed time. That is why an age of a certain number of years means at least that many and not approximately.
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How to use this calculator
- Date of birth. Enter your own figure; the page starts at 1990-06-15.
- Age at this date. Defaults to today when you open the page.
The result updates as you type, so you can try a figure, see what it does to age, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Date of birth | 1990-06-15 |
| Age at this date | 2026-09-08 |
What comes out
| Result | Value |
|---|---|
| Age main result | 36 years, 2 months, 24 days |
| Total days lived | 13,234 |
| Total months | 434 |
| Days to next birthday | 280 |
That is 36 years, 2 months, 24 days, or 13,234 days. The next birthday is in 280 days.
What each figure means
- Date of birth
- A number, starting at 1990-06-15.
- Age at this date
- Defaults to today when you open the page.
What the results tell you
| Result | What it is |
|---|---|
| Age | The headline result. A category or planning selection, rather than a measured quantity. |
| Total days lived | A number. |
| Total months | A number. |
| Days to next birthday | A number. |
Keep going
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