How it’s calculated
Age is counted the way people normally say it: completed years, then completed months, then the days since the last monthly anniversary of the birth date. If the birth day doesn't exist in a month (e.g. the 31st), that month's anniversary falls on its last day.
Example: born 15 May 1990, as of 4 October 2026. The last monthly anniversary is 15 September 2026, which is 36 years and 4 months after birth, and 4 October is 19 days later. Age = 36 years, 4 months, 19 days (13,291 days in total).
All dates use the Gregorian calendar.
Frequently asked questions
How is a February 29 birthday handled?
Age in years still increases each year. In common years we treat February 28 as the anniversary, so the age ticks over that day. Some laws and people use March 1 instead.
Why do month-and-day results sometimes look odd near month ends?
Months have different lengths, so "one month" after January 31 is ambiguous. We count from the last monthly anniversary, placing it on the month’s last day when needed (31 January → 29 February in a leap year).
Does it count leap days in the total?
Yes. Total days are the exact number of calendar days between the two dates.
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Sources
- Introduction to Calendars — U.S. Naval Observatory
- Julian Date Converter — U.S. Naval Observatory
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
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