Age Calculator

How the Age Calculator works

Age calculator: counting years, months and days

Ages look like simple subtraction, and they are not. Months have different lengths, leap days come and go, and browsers quietly shift dates across time zones. Most hand-rolled age calculations get at least one of those wrong. This calculator handles all three and gives your age three ways: as years, months and days, as total days lived, and as the countdown to your next birthday.

Why it matters. Exact age decides real things. School entry cut-offs, pension and retirement eligibility, junior and senior fares, insurance premiums, sports age groups and the legal ages for driving, voting and drinking all turn on completed years on a given date. Doctors track children's development in months and weeks. HR teams and solicitors need an age on a past or future date, not just today. Being out by one day at a threshold can mean a refused application.

Calculate age from date of birth: a worked example

Take a birth date of 17 April 1991 and a comparison date of 15 September 2026.

1. Years: 2026 − 1991 = 35.
2. Months: 9 − 4 = 5.
3. Days: 15 − 17 = −2. That is negative, so borrow the length of the month before September. August has 31 days, so 31 − 2 = 29 days, and the months drop from 5 to 4.

The exact age is 35 years, 4 months and 29 days. If the months had gone negative, the same borrowing would add 12 and take one off the years.

The total-days figure is worked out separately, by counting every calendar day between the two dates. In this example it is 12,935, including the nine 29 Februaries in the gap.

Month-end dates have no perfect answer

A month is not a fixed quantity, so the same number of days can be a different number of months depending on where it falls. From 31 January to 28 February, the calculator borrows January's 31 days and reports 28 days, not one month, because the 31st has not come round again. From 31 January to 3 March it borrows February's 28 days and reports 1 month and 0 days. There is no universally agreed answer to "one month after 31 January", which is why the result is whole months plus leftover days rather than a rounded month count. When you need a single unambiguous number, use the total days.

How many days have I been alive?

Days lived counts real calendar days, so every leap day in your span is included automatically. Your own figure will sit a day either side of these, depending on how many 29 Februaries fall between your birth and the birthday.

Table: days, weeks and hours lived at milestone birthdays

Round-number milestones fall at odd ages, which is why some people celebrate them: 10,000 days is about 27 years and 4 months, 20,000 days is 54 years and 9 months, and 30,000 days comes just after an 82nd birthday.

How to read the result.

• Age is completed years, months and days. The years figure is your legal age almost everywhere.
• Days lived is the exact count of days from birth to the comparison date. It is the most reliable figure for anything that needs precision.
• Next birthday in counts days to the next anniversary. On your birthday itself it shows a full year ahead, because today's birthday has already arrived.

If the birth date is after the comparison date, you get "That date has not happened yet" instead of a negative age.

Tips and pitfalls.

• Ages on other dates. Fill in "On this date" to find how old someone was at an event, or will be on a future deadline such as a school start date or retirement.
• Not just birthdays. The tool measures the gap between any two dates, so it works for how long you have been married, employed or living somewhere. For business days, use the working days calculator; for a plain gap in several units, use the date difference calculator.
• 29 February birthdays. In ordinary years the next-birthday figure treats 1 March as the anniversary, a common convention, so it can read a day later than expected. The years, months and days age is unaffected. Some countries legally use 28 February instead, so check where it matters.
• Time zones. A browser reading 1991-04-17 the obvious way treats it as midnight UTC, which is the previous day for anyone in the Americas. This calculator reads the year, month and day exactly as written and never converts between zones, so the answer does not depend on where you are or what hour it is.
• Legal thresholds. Most jurisdictions count completed years, but a few legal systems treat a person as reaching an age on the day before the birthday, and East Asian age reckoning historically counted differently. When a threshold matters legally, check the rule that applies.

For why date maths trips up spreadsheets and code, read date arithmetic that goes wrong.

Age formula and worked examples

The formula: age in years = the number of whole birthdays passed; the leftover months and days are counted forward from the last birthday. Days lived = target date − date of birth, counted in calendar days including leap days.

First example. Born 15 March 1990, on 1 October 2026 the age is 36 years, 6 months and 16 days, and the person has lived 13,349 days.

Second example. Born 1 January 2000, on 31 December 2025 the age is 25 years, 11 months and 30 days; one day later it is exactly 26, with 9,497 days lived.

How many days old am I? Subtract the date of birth from today's date in days; the table gives the figure for 1 January birthdays.

Age and days lived on 1 January 2026, by year of birth (born 1 January)

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