How does the leap-year birthday work?

Anniversaries follow real calendar months, so a Feb 29 birthday reaches its yearly mark on Mar 1 in non-leap years; the day-by-day breakdown stays exact either way.

Does my timezone change the result?

No. Dates are interpreted as calendar dates at UTC midnight, deliberately ignoring timezone offsets so results are reproducible everywhere.

Can I calculate an age at a past or future date?

Yes; fill in the optional as-of date. Future as-of dates give the age the person will have reached on that day.

Why do total days differ from the years/months/days line?

They measure different things: total days is the raw elapsed count, while the breakdown follows calendar borrowing rules where partial months contribute irregular day counts.

Dates & Time

Age Calculator

What this does

Work out an exact age in years, months and days from a date of birth, plus total days and weeks lived, using precise calendar arithmetic.

Enter your details

Runs in your browser

How to use it

Using the age calculator

01
Enter the date of birth
Use the YYYY-MM-DD format provided by the date picker.
02
Optionally set an as-of date
Leave it blank to use today, or pick any date to answer “how old was/will they be on…?”.
03
Read the breakdown
The headline gives exact years/months/days; the rows show total days and weeks lived.

Good to know

Why ages are trickier than they look

Months have 28–31 days and years occasionally gain a day, so “divide the day difference by 365.25” drifts within a few years. Calendar-aware counting; borrowing days from the actual previous month when the day-of-month falls short; matches legal and cultural conventions everywhere.

Leap-day birthdays

People born on February 29 legally age on March 1 in most jurisdictions during common years. This calculator follows that convention: on 2023-02-28 someone born 2000-02-29 is still 22, turning 23 only after Feb 29 exists again in leap years like 2024.

Determinism first

Because every computation is anchored to UTC midnight of the entered calendar dates, the same input always yields the same output; on your laptop, on a server, or inside an automated test.

How it's calculated

The math behind this calculator

Age = full calendar years between dates, plus leftover whole months, plus leftover days
Total days = (as-of − birth) ÷ 86,400,000 s

We count complete calendar years first, then complete months within the remaining period, then leftover days; exactly how people describe ages in everyday speech. Leap-year February 29 birthdays are handled by real calendar month lengths rather than a fixed 365.25-day year.

All arithmetic happens on UTC midnight timestamps built from the yyyy-mm-dd strings you enter, so no timezone or daylight-saving rule can shift a result by a day.

Assumptions & limitations

Dates are treated as pure calendar dates with no timezone shifting.
A Feb 29 birthday reaches its yearly mark on Mar 1 in non-leap years for anniversary counting.
The as-of date defaults to today when left blank.

Worked example

Someone born on 1990-06-15 was 36 years, 2 months and 8 days old as of 2026-08-23.

FAQ

Frequently asked questions

How does the leap-year birthday work?: Anniversaries follow real calendar months, so a Feb 29 birthday reaches its yearly mark on Mar 1 in non-leap years; the day-by-day breakdown stays exact either way.
Does my timezone change the result?: No. Dates are interpreted as calendar dates at UTC midnight, deliberately ignoring timezone offsets so results are reproducible everywhere.
Can I calculate an age at a past or future date?: Yes; fill in the optional as-of date. Future as-of dates give the age the person will have reached on that day.
Why do total days differ from the years/months/days line?: They measure different things: total days is the raw elapsed count, while the breakdown follows calendar borrowing rules where partial months contribute irregular day counts.

Keep exploring

Using the age calculator

Enter the date of birth

Optionally set an as-of date

Read the breakdown