Date & Time
Age Calculator
Enter a birth date and optional comparison date to calculate age.
Your details
Enter your details below
Your result
Based on the values you enter
Useful next checks
- Confirm timezone and calendar assumptions for important dates.
- Try nearby dates to compare different scenarios.
- Use the detail text to check exactly what was counted.
Intro
Use this age calculator to find completed years of age and the total calendar days between a birth date and a comparison date.
How it works
Enter a valid birth date and an optional comparison date. Leave the comparison date blank to use today's date in your device's local timezone. A comparison date may be the same as the birth date, but cannot be earlier.
Formula
Completed years are the difference between the calendar years, minus one if the comparison date is before the birth month and day in that year. This calculator shows completed years and total calendar days, rather than a separate years/months/days breakdown.
Total calendar days count date boundaries. They are not elapsed clock hours divided by 24: daylight-saving changes can make a local day shorter or longer. The day count includes leap days and does not count both endpoints inclusively, so the same date gives zero days.
The implementation validates the entered year, month and day, then compares calendar-day ordinals independently of local timezone offsets. This method was checked on 14 October 2026 against the ECMAScript date specification.
Example calculation
Someone born on 1 January 2000 is 30 years old on 1 January 2030, with 10,958 calendar days between those dates.
Across the UK spring clock change, 28 March 2026 to 30 March 2026 is 2 calendar days, even though those local midnights are only 47 hours apart. The autumn example, 24 October 2026 to 26 October 2026, is also 2 calendar days.
Assumptions and limitations
The result uses calendar dates without times of birth. The blank-date shortcut uses the device's local date, so people in different timezones may have different “today” dates at the same instant. An explicitly entered comparison date gives the same calendar result in every timezone.
For a 29 February birth, the existing month/day comparison reaches the next completed year on 1 March in a non-leap year: for example, 29 February 2000 to 28 February 2026 gives 25 years, and to 1 March 2026 gives 26 years. This describes the calculator's arithmetic; it does not determine a legal or clinical birthday rule.
Frequently asked questions
Does it account for leap years?
Yes. Valid leap days count towards the total calendar days, and impossible dates such as 29 February 2025 are rejected. Completed years use the month/day comparison described above.
Can I calculate age on a future date?
Yes. Enter a valid past or future comparison date on or after the birth date. The same birth and comparison date gives 0 years and 0 calendar days.
Can daylight saving change the day count?
No. The day count compares calendar dates independently of local UTC offsets. A spring or autumn clock change does not add or remove a calendar day.