Age Calculator

About Age Calculator

An age calculator finds your exact age from your date of birth — in years, months, and days — and shows how many days you've lived and how long until your next birthday. Enter a birthday to get a precise breakdown instantly, correctly accounting for leap years and differing month lengths, or measure the span between any two dates.

Why age arithmetic is harder than it looks

Subtracting two dates sounds trivial until you try it. Months have different lengths, so there is no fixed number of days in a month to divide by. Years have either 365 or 366 days. The result is that days, months, and years cannot be converted between each other cleanly, and any calculation that tries — dividing total days by 30.44 to get months, say — produces an approximation that drifts.

The correct method counts down the units in order. Take the whole years between the two dates, then the whole remaining months, then the leftover days. That gives a breakdown that always reconstructs the original dates exactly, which the divide-and-round approach does not.

The leap year rule in full

A year is a leap year if it is divisible by 4, except years divisible by 100, unless they are also divisible by 400. So 1900 was not a leap year and 2000 was. This catches out a surprising amount of software; a famous spreadsheet bug treats 1900 as a leap year to this day for backwards compatibility, which means date arithmetic before March 1900 is off by one in several widely used programs.

The rule exists because the earth's orbit takes about 365.2422 days, not 365.25. Adding a day every four years overcorrects slightly, so the century rules subtract the excess back out. The result is accurate to about one day in 3,000 years.

People born on 29 February

A leap-day birthday has no anniversary in three years out of four, and there is no universal answer to when it falls. Different jurisdictions have made different choices for legal purposes — some treat 28 February as the birthday in common years, others treat 1 March, on the reasoning that the person completes their year at the end of February.

For practical purposes most software picks 1 March in a common year, which keeps the sequence of days intact. If it matters legally — for a licence, a contract, or an age threshold — the local rule is what counts, not the calculator's convention.

The end-of-month ambiguity

There is a genuine ambiguity in adding months that no calculator can resolve for you. What is one month after 31 January? Both 28 February and 3 March are defensible: the first clamps to the end of the shorter month, the second adds the same number of days. Most systems clamp, which means the operation is not reversible — add a month to 31 January and subtract it again and you land on 28 January.

This is why durations expressed in months are unreliable for anything precise, and why contracts, subscriptions, and interest periods usually specify a day count or an explicit rule instead. When exactness matters, count days.

Frequently asked questions

How old am I exactly?

Enter your date of birth and the calculator returns your exact age in years, months, and days as of today, plus the total number of days you've been alive.

How is exact age calculated?

It counts full years, then the remaining months, then the leftover days — correctly handling leap years and months of different lengths — instead of just dividing total days by 365.

Can I find the difference between two dates?

Yes. Enter any start and end date to get the span in years, months, and days, along with the total number of days between them.

Does it handle leap years?

Yes. February 29 and varying month lengths are accounted for, so both your age and any date difference are accurate.