Age Calculator

Where age calculators get it wrong

• Subtracting timestamps. The lazy implementation is (now - birth) / 86400000. That is wrong twice over: a day is not always 86,400 seconds where daylight saving exists, so the answer drifts by a day twice a year, and it silently mixes the time of day into a calculation about dates. This tool works on year, month and day numbers and never touches a clock.
• Borrowing the wrong month. When the day of the month underflows — born on the 31st, measured on the 1st — you have to borrow the length of a specific month, and which one is not obvious. The right answer is: step back whole months from the birth date until you land on or before the target, clamping to the end of short months, then count the leftover days. Born 31 January, measured 1 March in a leap year, that gives one month and one day. Naive borrowing gives a negative number of days.
• Assuming a month is thirty days. "Total months" here counts real calendar months, not days divided by 30.44. The two disagree by a few days a year and the difference compounds.
• Counting from one instead of zero. Somebody born today is nought days old, not one day old. You are in your first year of life, not aged one.

29 February, and why this tool asks

A person born on 29 February has a real anniversary only once every four years. In the other three the date does not exist, so someone has to decide when they legally get a year older — and that is a matter of local law and local habit rather than arithmetic.

Two conventions are in circulation. One treats the anniversary as 28 February, which is what most software does and what most people do socially. The other treats it as 1 March, on the reasoning that the person has not completed the year until the last possible day of February has passed; England and Wales are usually described as taking this view for the purpose of attaining an age. The gap between the two is a single day, and it matters exactly once: on 28 February in a common year, when it decides whether someone is legally old enough to do something.

We are not lawyers and this page is not advice. If the day matters — a licence, a drinking age, an insurance band, a pension date — check what applies where you are rather than trusting any calculator, this one included.

What the extra numbers are

The hours and minutes figures are whole calendar days multiplied by 24 and 1,440. They are exact as counts of days; they do not know what time of day you were born, and this tool deliberately does not ask.

The Julian Day Number is the count astronomers use: one continuous stream of days with no months, no years and no calendar reforms, running from a zero point in 4713 BC. It is what you use when you need to subtract two dates thousands of years apart without arguing about which calendar was in force. Alongside it is the Unix day number, the same idea with its zero at 1 January 1970, which is what nearly every computer you own is counting in underneath.

One honest limitation

Dates before the Gregorian calendar was adopted are computed by running the Gregorian rules backwards — the proleptic Gregorian calendar. That is the standard convention and it is what ISO 8601 specifies, but it is not what a parish register from 1650 says. For any date before the 1750s, expect the written record and this calculator to disagree by several days.