Age Calculator
Your exact age in years, months and days — plus the total days, the next birthday, and the weekday you were born on.
Runs entirely in your browser. Nothing you enter is uploaded.
Written and maintained by ReadoutKit / Takashi Yamazaki
How this works
The part that is not subtraction
Working out someone's age in whole years is easy: take the difference in years, then knock one off if the birthday has not yet arrived this year. Expressing the leftover as months and days is where it stops being obvious, and where most hand-rolled calculations go wrong.
The temptation is to treat the date like a three-digit number and borrow between columns. It fails on real dates. Someone born on 31 January, measured on 1 March: the day column gives 1 − 31 = −30. Borrow February's 28 days and you still have −2. There is nothing left to borrow that produces a sensible answer, because the question itself is ill-posed — a “month” after 31 January does not land on a date that exists.
The method that works is to count forward instead of backward. Add whole months to the birth date until one more would overshoot, clamping to the last day when the target month is short, then measure what remains in days. 31 January plus one month is 28 February, and 1 March is one day after that: one month and one day. The same procedure gives one month and one day for someone born on 31 March measured on 1 May, even though April has thirty days rather than twenty-eight, which is the sanity check that the method is doing something principled rather than coincidental.
Adding months is not reversible
A consequence worth internalising: month arithmetic loses information. 31 January plus one month is 28 February. Subtract one month from that and you get 28 January, not 31 January. The clamping discarded three days and nothing gets them back.
This is why date libraries disagree with each other about month arithmetic and why contracts that say “one month from” often specify what happens at month ends. It is not a bug in any particular implementation; it is a property of a calendar whose months have different lengths. Any tool that presents month arithmetic as clean and reversible is hiding the problem rather than solving it.
Leap-day birthdays
Someone born on 29 February has a birthday that exists in roughly one year in four. Three years in four, the calendar offers no matching date, and a convention has to be chosen.
This calculator uses 28 February, and separately reports how many times the actual date has occurred. That second number is the one people are usually curious about, and it is smaller than they expect. Below, someone born on 29 February 2000, measured on five different days:
| Measured on | What the day is | Age | Real birthdays | Days lived |
|---|---|---|---|---|
| 2026-02-27 | Two days before | 25 y 11 m 29 d | 6 | 9,495 |
| 2026-02-28 | A common year: the anniversary lands here | 26 y 0 m 0 d | 6 | 9,496 |
| 2026-03-01 | The day after | 26 y 0 m 1 d | 6 | 9,497 |
| 2028-02-28 | A leap year: not yet | 27 y 11 m 30 d | 6 | 10,226 |
| 2028-02-29 | The real birthday | 28 y 0 m 0 d | 7 | 10,227 |
By 28 February 2026 this person is 26 by the ordinary reckoning and has had six actual birthdays. The seventh does not arrive until 29 February 2028, at which point they turn 28 — the count of real birthdays rises by one while the age rises by two.
Japanese law arrives at the 28 February result from a different direction. Under the statute governing the calculation of age, a person's age advances at the close of the day preceding the anniversary, which places the change inside February regardless of whether the 29th exists that year. Other systems specify 1 March. Where a legal threshold turns on the distinction — school entry, licensing, contractual majority — the governing rule is the one that matters, not the calculator.
Why the totals use days rather than averages
Total days and total weeks here are exact counts, computed by differencing calendar dates. Whole months is an exact count of completed monthly anniversaries. None of these figures is derived from an average month length. The day milestones show why that matters:
| Milestone | Falls on | Age that day |
|---|---|---|
| 10,000 days | 2017-10-31 | 27 y 4 m 16 d |
| 20,000 days | 2045-03-18 | 54 y 9 m 3 d |
| 30,000 days | 2072-08-03 | 82 y 1 m 19 d |
Ten thousand days is 27 years, 4 months and 16 days. Dividing by 365.25 gives 27.38 years, and nothing in that decimal tells you what the remainder is in months and days, because it depends on which months were crossed. The exact answer needs the calendar, not an average.
The same distinction matters for anyone reconciling this against another tool. A total-days figure divided by 30.44 will not equal the whole-month count — for a 36-year-old it lands about six-tenths of a month out — and neither is wrong; they answer different questions. The average is a useful approximation across long spans and a poor one across short ones.
Time zones, and why they are absent here
A date of birth is a calendar date, not an instant. Attaching a time to it introduces problems that have no business being in an age calculation: intervals that change length across daylight-saving transitions, and results that differ depending on where the person asking happens to be sitting.
Everything here operates on year, month and day alone, anchored to UTC internally so that no local offset can shift a boundary. The output is the same in Auckland and in Los Angeles, which is the correct behaviour for a question about dates. It also means the result is stable if you reload the page in a different place, and that nothing about your location is needed — or transmitted — to produce it.
Common questions
- When does someone born on 29 February have a birthday?
- This calculator advances the age on 28 February in common years, and separately counts how many times the real date has occurred. Someone born on 29 February 2000 is 26 on 28 February 2026, but has had only six actual birthdays by then; the seventh arrives on 29 February 2028, when they turn 28. Japanese law reaches the same February result from a different direction, because age advances at the close of the day before the anniversary. Some other systems specify 1 March instead, so where a legal threshold depends on it, the governing rule decides — not a calculator.
- When will I be 10,000 days old?
- Ten thousand days after a birth date on 15 June 1990 falls on 31 October 2017, when that person is 27 years, 4 months and 16 days old. The reason a calculator is needed is that dividing 10,000 by 365.25 gives 27.38 years and stops there: the remaining 0.38 of a year is a different number of days depending on which months it crosses. The tool counts real calendar dates, so the answer holds for any birth date rather than only for an average year.
- Why can age not be calculated by subtracting the numbers?
- Because months are not the same length, so the day and month columns cannot simply be borrowed from like decimal digits. Take someone born on 31 January, measured on 1 March. Subtracting gives −30 days, and borrowing 28 days from February still leaves −2. Every method that works instead counts whole months forward from the birth date and then measures the remainder in days. That gives one month and one day, which is what people mean.
- What happens to a 29 February birthday in a common year?
- There is no correct answer, only conventions. This calculator treats 28 February as the anniversary in common years, and shows separately how many times the real date has come round — roughly once every four years. Japanese law reaches the same practical result by a different route: age increases at the end of the day before the birthday, so a leap-day birthday still advances during February. Some jurisdictions and institutions use 1 March instead. If a legal deadline depends on it, check the rule that actually applies to you.
- Why does the total number of months not match years times twelve plus months?
- It does — that is exactly how it is calculated here. What it will not match is the total days divided by 30, or by 30.44. Those are averages, and no individual month is that long. Whole months is an exact count of completed monthly anniversaries; anything derived from an average day-count is an approximation that drifts by up to a day and a half per month.
- Does the result change depending on my time zone?
- No. All arithmetic here is done on plain calendar dates with no time attached, fixed to UTC internally. This matters more than it sounds: subtracting two local timestamps across a daylight-saving change produces intervals of 23 or 25 hours, and a division that truncates will report one day fewer than the calendar shows. Dates are counted as dates, so the answer is stable wherever you are.
- Can I work out an age at a past or future date?
- Yes — that is what the second field is for. It defaults to today, but setting it to any other date gives the age on that date. This is the usual reason people reach for an age calculator in the first place: eligibility on an enrolment date, age at a past event, or how old someone will be when a policy matures.
Sources
The formulas and rates used by this tool come from the documents below. Rules change; when a figure here disagrees with the source, the source is right.
Figures checked against the sources below on September 1, 2026.
- ISO 8601 — Date and time formatInternational Organization for Standardization
- RFC 3339: Date and Time on the Internet — TimestampsIETF