How Age Is Calculated, Including Leap Days

The short answer

That is why age cannot be derived from a single day count. 13,257 days do not divide cleanly into years and months, because the lengths of both depend on which years and which months you happened to pass through.

Counting it by hand

1. Count whole months first. Work out the number of months from the birth date to the target date: (target year − birth year) × 12 + (target month − birth month).
2. Test the anniversary. Take the birth date and move it forward by that many months, keeping the same day number. If the resulting date is later than the target date, the last month is not complete, so subtract one month.
3. Split the months. Whole months divided by 12 gives the years; the remainder gives the months.
4. Count the leftover days. Measure the plain day difference between the last monthly anniversary and the target date.

Example: born 15 June 1990, measured on 1 October 2026

Months from June 1990 to October 2026 = (2026 − 1990) × 12 + (10 − 6) = 436. Move 15 June 1990 forward 436 months and you get 15 October 2026, which is after the target, so drop to 435 months — 15 September 2026. That is 36 years and 3 months (435 ÷ 12 = 36 remainder 3). From 15 September to 1 October is 16 days. The answer is 36 years, 3 months and 16 days.

On the birthday itself the method gives 36 years, 0 months, 0 days; the day before, it gives 35 years, 11 months and 30 days.

The month-end rule

Step 2 asks you to keep the same day number, but the 31st does not exist in every month. The rule used by this site's calculator is to clamp to the last day of the shorter month: one month after 31 January is 28 February in a common year, 29 February in a leap year, and one month after 31 March is 30 April.

Take someone born on 31 January 2000:

The third row surprises people: a 30-day leftover. It is correct under the clamping rule, because the February anniversary landed on the 28th and March has 31 days. The alternative convention — letting 31 January roll forward past the end of February to 3 March — would put the anniversary three days later and report 26 years, 1 month and 27 days for that same target. Neither is wrong; they are different definitions, and the gap only ever appears for birth dates on the 29th, 30th or 31st.

Birthdays on 29 February

A person born on 29 February has a real birthday only in leap years. Legal systems differ on which day stands in for it during the other three: some jurisdictions treat 28 February as the day a year is completed, others treat 1 March, and some statutes say nothing, leaving it to case law or administrative practice.

This site's calculator applies the same clamping rule it uses everywhere else, which means 28 February is the anniversary in a common year. For a birth date of 29 February 2004:

If a form, contract or official process matters to you, use the rule that the relevant authority uses rather than this one. The calculator is a calendar tool, not a statement of law.

Why leap years exist at all

A year is close to 365.2422 days, so adding one day every four years overshoots slightly. The Gregorian correction skips the leap day in century years not divisible by 400: 1900 and 2100 are common years, 2000 was a leap year. Between 15 June 1990 and 1 October 2026 there were nine leap days, which is why that span is 13,257 days rather than the 13,248 it would be if every year had 365.

Why two calculators can differ by a day

• Month-end handling. Clamping 31 January to 28 February, as described above, or rolling it forward into March, gives different day remainders.
• 29 February convention. The 28 February and 1 March conventions disagree for one day each common year.
• Inclusive or exclusive day counts. Some tools count the birth day itself as the first day lived, adding one to every day total.
• Time zones. A tool that compares timestamps rather than calendar dates can be a day out whenever the two dates fall in different offsets. This calculator takes the plain calendar date from your device and compares dates only, with no clock time involved, so it does not drift.
• Borrowing order. Code that subtracts days first and borrows from the previous month can land a day away from code that counts whole months first, as this calculator does.
• Average-length months. Anything that divides a day count by 30.44 to produce months is an approximation and will disagree with calendar counting most of the time.

Common mistakes

• Subtracting the years and stopping. 2026 − 1990 is 36 only after 15 June; before that the person is still 35.
• Treating a month as 30 days. It produces a plausible-looking figure that drifts further the longer the span.
• Entering dates in the wrong order. A birth date later than the target date has no meaningful age; the calculator reports that the date of birth cannot be in the future rather than printing a negative.
• Mixing date formats. 03/04/2026 is March or April depending on where you are; the date fields avoid this by storing a year-month-day value.
• Assuming the leftover days are always under 30. A 30-day remainder is possible, as the table above shows. A 31-day one is not — if you see 31, something has been counted twice.

Check it yourself

Open the age calculator, enter 15 June 1990 and set the second field to 1 October 2026 to reproduce the example: it reports You are 36 years, 3 months, and 16 days old. Leave that field empty and it uses today's date from your device instead. Enter 29 February 2004 with 28 February 2026 for the leap-day rule, and 31 January 2000 with 30 March 2026 to see the month-end clamp give 26 years, 1 month and 30 days.

Because the second field accepts past dates too, it also answers "how old was I when…" questions and measures the gap between any two dates. It takes any date from year 1 onwards and rejects impossible ones such as 31 February.

Related tools and guides

• Age calculator
• Time zone converter
• Content calendar generator
• How a loan EMI is calculated

Reviewed October 1, 2026. Calculation methods and corrections.