Age Calculator
Work out an exact age in years, months and days from a date of birth — plus the totals in weeks and days, and how long until the next birthday.
Your data never leaves your browser. This tool runs entirely on your device.
Choose a date of birth and the date to calculate the age on.
How age is counted
Age is the number of complete years lived. You turn 34 on your 34th birthday, not during the year leading up to it, so the birthday must have passed in the current year for it to count.
The remaining months and days are counted forward from the last birthday, using real calendar months rather than an average of 30.44 days. That is why the same gap in days can come out as different numbers of months depending on which months it spans.
Month arithmetic and clamping
Adding a month to 31 January cannot give 31 February, so the day is clamped to the last day of the target month — 29 February in a leap year, 28 otherwise. Every mainstream date library behaves this way, and so does this calculator.
The consequence is that month arithmetic is not always reversible: 31 January plus one month is 28 February, but 28 February minus one month is 28 January.
Leap years and 29 February birthdays
A leap year is any year divisible by 4, except centuries, unless they are divisible by 400. So 2000 was a leap year and 1900 was not — a rule that catches a lot of software out.
Someone born on 29 February has a birthday only every four years. With the clamping convention used here, their birthday falls on 28 February in common years. Some jurisdictions use 1 March instead, so legal age of majority can differ by a day.
Why this works in UTC
All dates are handled in UTC. Local-time arithmetic goes wrong across daylight-saving boundaries, where a day can be 23 or 25 hours long, which would make a day count off by one twice a year.
Frequently asked questions
How is exact age calculated?
Count the complete years since birth, then the complete months since the last birthday, then the remaining days. The birthday must have already occurred this year for the year to count.
When does someone born on 29 February have a birthday?
Every four years strictly speaking. In common years this calculator treats the birthday as falling on 28 February, which is the standard clamping convention. Some legal systems use 1 March instead.
Which years are leap years?
Years divisible by 4, except those divisible by 100, unless also divisible by 400. So 2024 and 2000 are leap years, but 1900 and 2100 are not.
Why does the number of days not divide evenly into months?
Because calendar months have between 28 and 31 days. The months and days shown are counted against the real calendar, not against an average month length, so they are exact rather than approximate.
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