Age & Birthday Calculator

How it works

Enter the birth date and reference date as complete YYYY-MM-DD values. Use today fills the reference from your device’s local calendar date; it does not change the birth date. Dates are validated before calculation. February 30, a missing day, year zero and a reference before the birth date produce a clear error. Years 0001–0099 retain those exact years instead of being interpreted as dates in the twentieth century. Supported years are 0001–9999, using Gregorian leap-year rules throughout. Dates before a country adopted the Gregorian calendar are therefore interpreted using that same calendar, not reconstructed historical local dates.

To find completed months, the calculator first builds a date that is the proposed number of months after the original birth date. It keeps the original day number when possible and otherwise uses the last day of the target month. If that anchor is later than the reference date, one month is removed. Completed months are then divided into groups of twelve for the displayed years and months; remaining days run from the final anchor to the reference date. For a birth date of January 31 and reference date March 1 in 2026, February 28 is the one-month anchor, leaving one day. The answer is zero years, one month and one day.

Anchoring to the original date matters across multiple months. January 31 to March 30 is one completed month plus thirty days, because the two-month anchor is March 31. The tool does not carry February’s shorter day number into later months. For February 29 births, February anchors in a non-leap year follow your selected anniversary rule. A 2024-02-29 birth evaluated on 2025-02-28 is one year old with the February 28 rule, but zero years, eleven months and thirty days with the March 1 rule. Neither setting is silently treated as a universal official convention.

Total days are calculated from whole calendar dates, excluding the birth date. The same date has a difference of zero. Daylight-saving transitions cannot introduce fractions of a day, because local elapsed hours are not used. Weeks divide the total by seven and retain the remainder. The tool deliberately does not label total days multiplied by twenty-four as exact elapsed hours; dates alone cannot establish a birth time, time zone or elapsed duration.

The next birthday is the first matching anniversary on or after the reference date, so a birthday on that date has zero days remaining. The selected leap-day convention applies to the next birthday and any milestone. Entering an optional whole milestone age calculates that anniversary and reports whether it falls before, on or after the reference date. A milestone outside the supported year range is rejected. If only the next birthday would exceed year 9999, the existing age remains available and the birthday is marked outside the range.

Editing any date or option clears the previous answer until you calculate again. Copy includes the selected convention, and CSV preserves the inputs, completed months, anchor, total days, birthday and milestone. These are calendar facts under stated assumptions; they do not establish school admission, legal adulthood or any other eligibility rule.

Common use cases

• Checking calendar age on a past or future date
• Planning an upcoming birthday or personal age milestone
• Comparing February 28 and March 1 leap-day conventions
• Exporting an age calculation with its dates and assumptions
• Checking a month-end date calculation without negative day remainders

Frequently asked questions

Standards and limitations

• U.S. Naval Observatory: Gregorian leap-year rule
• MDN: UTC full-year date operations