How Age Is Calculated Precisely
Working out someone's age sounds trivial — but leap years, uneven months, and time zones all sneak in. This guide explains how an accurate calculation actually works.
The basic formula
Age in complete years is the difference between the current year and the birth year, minus one if the birthday has not yet occurred this calendar year.
For precision, we also break the remainder into months and days from the last birthday to today.
Handling months and days
Months are not equal in length, so subtracting them naively can produce negative days. A correct algorithm borrows days from the previous month, exactly like long subtraction.
Leap years and 29 February
People born on 29 February celebrate a legal birthday every year, but the exact date returns only in leap years. Most calculators anchor their birthday to 28 February or 1 March in non-leap years.
Time zones matter for the youngest ages
A baby born at 23:30 in Tokyo and measured against a server in New York can appear a full day younger. For everyday use this is harmless, but for medical or legal purposes always calculate in a single fixed timezone.