Age in Years
Whole years elapsed since a birth date.
Formula
Variables
Example
10,000 days old ⇒ about 27.4 years
How it works
This formula converts the number of days between a person's birth date and today into years, then takes the whole-number part. The divisor 365.2425 is used because a year is not exactly 365 days; it averages leap years over the long term. The floor function removes any partial year, so the result is the completed age in whole years.
When to use it
Use this when you need a person's age in full years from two dates, such as in forms, eligibility checks, or age-based school or sports categories. It is also useful in date calculations where only completed years matter, not months or days.
Worked example, step by step
Suppose a person was born on 2001-09-18 and today is 2026-01-10. JDN_today - JDN_birth = 8907 days 8907 ÷ 365.2425 = 24.386... floor(24.386...) = 24 Age in years = 24 years
Common mistakes to avoid
- Using 365 instead of 365.2425 and gradually drifting away from the true age
- Rounding the decimal result instead of taking the floor, which can overstate age before the birthday occurs
- Forgetting that age in whole years changes only on the birthday, not at the start of the calendar year
Did You Know?
The mean Gregorian year is 365.2425 days — that fractional bit is exactly what leap years exist to absorb.
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