How to use this calculator
Enter a birth date and the date you want to measure age on. Example dates are shown initially; Use today fills the as-of date using your device’s local calendar date. The result shows years, remaining months and days, total elapsed days and the next birthday.
The math behind it
Completed months = whole calendar months from birth date Years = floor(completed months ÷ 12) Remaining months = completed months mod 12 Days = as-of date − last completed monthly anniversary
A worked example
From January 15, 2000 to March 20, 2025, the age is 25 years, 2 months and 5 days. A birth date of February 29, 2020 reaches its first anniversary on February 28, 2021 under this calculator’s convention.
Assumptions & precision
Uses the Gregorian calendar for years 0001–9999. The birth date cannot follow the as-of date. Whole months are anchored to the original birth day; when that day does not exist in a month, it is clamped to the last day. February 29 yearly anniversaries use February 28 in non-leap years. For example, January 31 to March 30 is 1 month and 30 days. Calendar-day counts ignore time-of-day and daylight saving. A birthday on the as-of date is shown as today. A next birthday beyond year 9999 is unavailable. This is a calendar calculation, not a legal-age determination.
A common question
How are February 29 birthdays handled?
In a non-leap year, this calculator uses February 28 for the anniversary. In a leap year it uses February 29. Other conventions exist, so this is not a legal birthday rule.