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Age Calculator: full years, months, and days
Age is the time between a birth date and another date. The age calculator counts full years first, then the remaining months and days.
Imagine walking across calendar stepping stones. You step through full birthdays first, then count the leftover month and day stones.
Core Formula
This looks simple, but calendars are not perfectly even. Months have different lengths, and leap years add February 29.
| Part | What it checks | Why it matters |
|---|---|---|
| Full years | How many birthdays passed | Gives the main age |
| Months | Months after the last birthday | Months do not all have the same days |
| Days | Days after the last full month | Gives the exact leftover count |
| Leap year | Whether February has 29 days | Affects dates around February |
Step-by-step example
Find the age from March 10, 2010 to June 28, 2026.
- Count full birthdays from March 10, 2010 to March 10, 2026: 16 years.
- Count full months from March 10 to June 10: 3 months.
- Count leftover days from June 10 to June 28: 18 days.
- The age is 16 years, 3 months, and 18 days.
Leap-year rule
That is why 2000 was a leap year, but 1900 was not.
Common mistakes and edge cases
- February 29 birthdays need special handling. Not every year has February 29.
- The end date cannot come before the birth date. The calculator should show a clear warning.
- Exact age in hours can depend on time zone. Calendar age and clock age are not always the same.
- Months are uneven. Counting 30 days as one month can give the wrong calendar age.
Sources and accuracy notes
The Gregorian leap-year rule affects date and age calculations. See U.S. Naval Observatory: Leap Years and NASA: Calendar calculations.
CalculTools uses calendar dates for years, months, and days. Exact age by hour, minute, or second may vary by time zone.