How age is actually calculated, leap days included

Age looks like the simplest calculation on any form, and it is where a surprising number of systems get an off-by-one. The reason is that age is not a duration divided into years. It is a count of birthdays that have already happened.

The method that is actually correct

Take the difference in calendar years between the birth date and today, then subtract one if today is earlier in the year than the birthday. Someone born on 3 September 1990, on 1 September 2026, is 35, not 36, because their birthday has not arrived yet this year. Working from whole calendar units like this is right by construction and needs no correction factors.

The tempting shortcut is to count the days between the two dates and divide by 365.25. It is close, and it is wrong near birthdays: the fractional year drifts, and the answer flips a day early or late depending on where leap years fall. For anything with legal or medical consequences that is not good enough.

The leap day question

Someone born on 29 February has a birthday that exists in roughly one year in four. In common years, systems and laws differ on whether they age on 28 February or 1 March, and both conventions appear in real legislation around the world. There is no universal answer, so a calculator should tell you which convention it applied rather than pretending the question does not arise.

Age in months and days

For infants, age is usually wanted in months and days, and here the ambiguity returns. Adding one month to 31 January gives a date that does not exist, so implementations clamp to the last day of February or roll over into March, and those two choices differ by up to three days. When the number matters, such as for vaccination schedules, follow whatever convention the schedule itself specifies rather than trusting a general-purpose function.

Which "today" are you using?

Age depends on the current date, and the current date depends on where you are. A person in Auckland has already reached their birthday while it is still the previous day in London. Systems that compute age in UTC will therefore disagree with the person holding the cake, by a day, for part of the day. If the precise moment matters, decide explicitly whose local date counts.

Practical rules

• Count completed years by comparing calendar dates, never by dividing days.
• State the leap-day convention when the birth date is 29 February.
• Use the local date of the person concerned when the boundary matters.
• For legal thresholds, check the jurisdiction rather than assuming the common rule.

The age calculator works from calendar dates, so it changes on the birthday itself rather than approximately near it, and it shows the years, months and days separately so you can see exactly where the boundary falls.