Age Calculator

How an exact age is worked out

Most people can say their age in years without thinking. The awkward part is the months and days, because months are not the same length and years are not the same either.

This calculator counts whole months first, then the days left over. From 15 May 1990 to 26 September 2026 is 436 whole months — 36 years and 4 months — and then 11 more days. Writing it as “36.36 years” would be tidier and less useful, because nobody thinks in decimal years.

The alternative approach, subtracting the day, month and year separately and borrowing when a number goes negative, produces odd answers at month ends. It is the reason some calculators say 31 January to 1 March is “minus two days”.

Why days never divide neatly by 365

A year is 365 days, except when it is 366. The rule:

• Divisible by 4 → leap year
• Unless divisible by 100 → not a leap year
• Unless divisible by 400 → leap year after all

So 2024 was a leap year, 2100 will not be, and 2000 was. The correction exists because the Earth takes about 365.2422 days to orbit the Sun — slightly less than 365.25, so a leap day every four years overshoots by roughly three days every four centuries.

This is why an age in days is always counted from the actual calendar rather than multiplied out.

Birthdays on 29 February

Someone born on 29 February has a real birthday roughly once every four years. For the other three, a convention is needed, and there is no single worldwide answer.

• This calculator uses 1 March, which is the most common legal treatment: the next year of age is reached once February has ended.
• Some organisations use 28 February instead, so the birthday stays inside the same month.
• For counting completed years, both give the same age on 1 March; they differ only for one day.

The calculator marks leap-day birthdays explicitly so you can see which rule is being applied.

Uses beyond curiosity

• Eligibility dates. School entry, driving, voting, pension age — set the second date to the deadline and read the age directly.
• Contracts and notice periods. Length of service in years, months and days is often what an employment calculation needs.
• Medical forms. Infant age is usually asked for in weeks or months rather than years.
• Documents. Passport and visa rules often turn on age at the date of travel, not the date of application, which is exactly what the second field is for.

A note on precision

Age in hours is days × 24. Without a time of birth, nothing finer is honest. And across a lifetime, daylight-saving changes add and remove an hour here and there, which no calculator that asks only for a date can know about.