/alder

Age Calculator - How Old Am I & Days Between Dates

Calculate exact age on a chosen date, or the number of days, weeks, and months between two dates.

How old?

Days between dates

About the calculator

Age is calculated as whole years, months, and days up to the date you choose (today by default). Leap years are handled automatically. “Days between” counts the number of whole days between the two dates.

About this tool

The age calculator tells you exactly how old you (or someone else) are on a given date, in years, months and days. It also has a simple "days between two dates" tool. Useful for applications that need an exact age, planning anniversaries, milestones for children, working days to a deadline, or for the classic "how old was I when...?" question. All computed locally in your browser, no dates sent to any server.

How to use it

  1. Enter the date of birth in the first module.
  2. Set the "on date" (defaults to today).
  3. Read the age in years, months and days.
  4. Use the second module for days between any two dates.

Examples

Age on a given date
InputBorn 1985-04-12, on date 2025-11-20
Output40 years, 7 months, 8 days
Days between two dates
InputFrom 2024-01-01 to 2025-01-01
Output366 days (2024 was a leap year)
The calculator handles leap years automatically. It uses the Gregorian calendar, standard across Europe since 1582 (and in Britain since 1752).

Common use cases

  • Filling in applications and government forms that need exact age.
  • Calculating age on interview date or application deadline.
  • Milestones for children (100 days, 1000 days, first half-year).
  • Working out seniority, work anniversary or pension age.
  • Comparing two people’s ages or age difference.

Frequently asked questions

How is age calculated when the birthday is 29 February?
In non-leap years, 29 February birthdays fall on 28 February or 1 March depending on country and context. The calculator uses the common "next existing date" interpretation, so 29-02-2000 → 01-03-2001 (365 days, age 1 reached on 1 March). This is handled differently in law: Norway’s population register treats 28 February as the birthday in non-leap years, but most other systems pick 1 March.
Are the numbers accurate?
Yes. The calculator uses JavaScript’s built-in Date handling, which is millisecond-accurate and correctly handles leap years, uneven month lengths and the Gregorian calendar. It doesn’t treat daylight-saving or time zones specially (since we only count whole calendar days), so results are identical wherever you are.
Can I get my age on Mars or in another calendar?
No, the tool uses only the Gregorian calendar and Earth time. For Mars years divide Earth-years by 1.881; Hebrew or Islamic ages need their own conversions (Islamic year is ~354 days; Hebrew is lunisolar with leap months). For "age in seconds" use days × 86,400.
Are my dates private?
Yes. All calculations happen in your browser in JavaScript. No birth dates, results or other data are sent to any server, and we store nothing. You can even use the tool offline after the first load.

Technical background

Age is computed from the difference between two JavaScript Date objects. For "years, months, days" we start with year difference, adjust down if the month component hasn’t been reached yet, then compute remaining months and days by stepwise subtraction of calendar values. The naive alternative (dividing total days by 365.25) is wrong near year boundaries. The leap-year rule (divisible by 4, except century years not divisible by 400) is handled by the Date class itself. Precision is milliseconds, but we round to whole days for readability. All logic runs in the browser; no data is serialised or sent out.