Age Calculator

Method: the calculator subtracts date components in order — days, then months, then years — borrowing from the next larger unit whenever a subtraction would go negative. In outline: days = asOf.day − dob.day; if negative, subtract one month and add the number of days in the previous month; months = asOf.month − dob.month (after the day borrow); if negative, subtract one year and add 12; years = asOf.year − dob.year (after the month borrow). The "days in previous month" figure is computed dynamically using JavaScript's new Date(year, month, 0) trick, which returns the last day of the month before the given one — this automatically accounts for 28, 29, 30, or 31-day months without a lookup table. Worked example 1: date of birth 3 June 1990, reference date 18 July 2026. Days: 18 − 3 = 15 (no borrow needed). Months: 7 − 6 = 1 (no borrow). Years: 2026 − 1990 = 36. Result: 36 years, 1 month, 15 days. Total days lived: 13,192. Worked example 2 (borrowing across a leap day): date of birth 29 February 2000, reference date 1 March 2026. Days: 1 − 29 = −28, so borrow a month — the month before March 2026 is February 2026, a non-leap year with 28 days, giving 1 − 29 + 28 = 0 days, and the month count drops by one before its own subtraction. Months: 3 − 1 − 1 = 1, then re-subtracting against the birth month (2) gives 0 months after the borrow resolves. Years: 2026 − 2000 = 26. Result: 26 years, 0 months, 0 days — meaning 1 March 2026 lands exactly on the 26-year mark from the 29 February 2000 birthday, handled gracefully even though 2026 has no 29 February of its own. Edge cases handled: - Date of birth after the "As Of" date: the calculator shows an explicit error ("Date of birth must be before the reference date") instead of a nonsensical negative age. - Leap-year birthdays (29 February): the borrowing logic always asks the calendar what the previous month actually contained, rather than assuming 30 days, so it resolves correctly even in non-leap reference years. - Same-day birthdays: if the reference date matches the day and month of birth exactly, the result is a whole number of years with 0 months and 0 days. - Very old or far-future reference dates: the arithmetic works identically regardless of the year range, since it operates on real Date objects rather than fixed-precision counters. Practical tip: if you need an age for a legal or administrative purpose (school enrolment cut-offs, insurance eligibility, employment minimum age), always double-check whether the relevant rule uses "age on the day of the event" or "age at the start of the year" — these can differ by up to a year for anyone whose birthday falls later in the year than the cut-off date, and this calculator's "As Of" field lets you test both scenarios directly by entering the exact cut-off date instead of today.