Date of Birth

Calculate As Of (optional)

Enter a date of birth and get age in years, plus a full years/months/days breakdown and a countdown to the next birthday. An optional as-of date lets you compute age at any past or future reference point instead of today.

How February 29 birthdays are handled

The calculator walks the calendar forward from the birth date, adding whole years, then whole months, then counting leftover days. This avoids the classic borrow bug where subtracting day-of-month values goes negative and the code borrows a month of the wrong length.

One consequence worth knowing: if you were born on February 29, this implementation treats March 1 as your anniversary in every year — including leap years. Born February 29, 2000 and checking as of February 28, 2025, you get 24 years, 11 months, 27 days with the next birthday 1 day away (March 1). Check again as of March 1, 2025 and the age ticks over to exactly 25 years, 0 months, 0 days. Jurisdictions split on this — New Zealand deems leapling birthdays to fall on February 28, most others on March 1 — so if you are checking legal eligibility for a leapling, confirm the local convention rather than trusting any calculator.

The as-of date is more useful than it looks

Leaving the second field blank computes age as of today. Filling it in answers questions like "how old was the applicant on the filing date" or "will this child be 5 by the school cutoff" — the countdown to the next birthday also re-anchors to the as-of date, so the two figures always agree with each other. The only hard validation is direction: a birth date after the reference date is rejected outright rather than producing a negative age.

A 31st-of-the-month birth date crossing February

Input: Born 1990-01-31, as of 2024-03-01

Output: 34 years, 0 months, 30 days — next birthday in 336 days

The month walk cannot land on February 31, so the span past the January 31 anniversary is expressed purely in days. A naive day-of-month subtraction would report 1 month here.

Good to know: Both date fields accept Enter to calculate, so you can tab between them and never touch the mouse. For the distance between two arbitrary dates that are not a birth date and a reference date, the date difference calculator is the better fit — it also reports totals in weeks and days.

The date difference calculator uses the same calendar-walking breakdown for any two dates, and the timestamp converter helps when your source data stores birth dates as Unix epochs.