Processed entirely on your device — nothing is uploaded
How to use the age calculator
- 1Enter the date of birth, or the start date.
- 2Set the end date — it defaults to today.
- 3Read the age in years, months and days.
- 4Check the totals in months, weeks, days and hours below.
Why calendar arithmetic is harder than it looks
Months do not have a fixed length, so "one month later" is not a fixed number of days. Adding a month to 31 January has no obvious correct answer, and different software gives different results — some say 28 February, some 3 March.
This calculator uses the convention almost everyone means by age: count whole years first, then whole months, then remaining days, borrowing from the preceding month when the day-of-month is smaller in the end date. Someone born on 15 March 1990 is 34 years, 2 months and 10 days old on 25 May 2024.
Leap years add another layer. There is one every four years, except century years, except century years divisible by 400 — which is why 2000 was a leap year and 1900 was not. Counting days between dates has to account for all of that, and this tool does.
The 29 February problem
People born on 29 February have a birthday that exists roughly once every four years, which creates a genuine ambiguity about when they legally turn a given age in non-leap years.
Different jurisdictions resolve it differently. Some treat 28 February as the birthday, some 1 March. The distinction rarely matters, but it can for legal thresholds — reaching the age of majority, or eligibility dates.
This calculator counts elapsed time rather than applying any legal rule, so it gives a consistent answer regardless of jurisdiction. If a precise legal date matters, check the rule that applies where you are.
Uses beyond birthdays
The same calculation answers a lot of practical questions. How long you have been in a job, for a CV or a long-service entitlement. How long a contract has run, or how long until it expires. Gestational age. The gap between two historical events. Time until a deadline.
The running totals are often the more useful output. Visa and residency rules are frequently expressed in total days present rather than in years. Probation and notice periods are usually in weeks. Project timelines are in working days, for which the total-days figure is the starting point.
The next-anniversary figure at the bottom answers the other common question directly: how long until this date comes round again.
Time zones and the boundary cases
Dates here are handled as plain calendar dates without time components, which is almost always what you want when calculating an age. It also avoids the class of bug where a date shifts by one day because of a time zone conversion.
The consequence is that the result does not depend on where you are or when you load the page, beyond the default end date being your local today. Two people in different countries entering the same two dates get the same answer.
If the end date is before the start date, the tool reports the absolute difference and says so, rather than returning a negative or refusing — which is usually what someone wants when they have entered the dates in the order they happened to think of them.
Frequently asked questions
How is age calculated exactly?
Whole years first, then whole months, then remaining days, borrowing from the preceding month when needed. This matches how people normally state an age.
Does it handle leap years?
Yes, including the century rules — 2000 was a leap year, 1900 was not.
What about someone born on 29 February?
The tool counts elapsed time consistently. Which day counts as the legal birthday in non-leap years varies by jurisdiction.
Can I calculate a duration between two past dates?
Yes. Set both dates to whatever you need; neither has to be today.
What if I enter the dates the wrong way round?
It reports the absolute difference and tells you the dates were reversed, rather than returning a negative.
Is my date of birth stored?
No. The calculation runs entirely in your browser and nothing is transmitted.