How to Calculate Age

People usually want two answers: “how old am I in completed years?” and “how long is that in months or days?” Those are different measurements. Completed years follow birthday anniversaries. Total days is a simple subtraction of calendar dates.

The calendar method

Start with the later date (today, or any “age at” date) and subtract the date of birth. If the day-of-month has not been reached yet, borrow one month and add the number of days in the previous month. If months go negative, borrow a year. That is what the Age Calculator on this site does.

1. Write both dates as year, month and day.
2. Subtract days. If the result is negative, reduce months by one and add the length of the previous month.
3. Subtract months. If the result is negative, reduce years by one and add 12 months.
4. The leftover years, months and days are the age.

Worked example

Born 15 March 2000, measured on 15 September 2026: the day-of-month matches, so leftover days are 0. Months are 6. Years are 26. Total days is the UTC date difference (9,680 days in this example), and total weeks are that number divided by 7, discarding the remainder.

Daylight-saving changes should not change a civil age. Counting at midnight in local calendar dates, or using a UTC date difference for total days, avoids “23-hour days”.

What this is not

• It is not a legal-age opinion for contracts or driving licences in a specific country.
• It does not use 30-day months. February is 28 or 29 days.
• It will not invent an age if the “age at” date is before the date of birth.