How To Calculate Age Between Dates
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The Y/M/D decomposition
Calculating age between two dates sounds simple — subtract the years — but the edge cases matter. Leap years, inclusive versus exclusive counts, whether “23 years old” means completed years or the current year of life, legal age jurisdictions that differ by purpose, pregnancy counted in weeks versus months. This guide walks through the math, the conventions you’ll run into, and where the gotchas actually bite.
Leap years — the February 29 trap
The standard way to express age between two dates: years, months, days — the largest unit first, carrying down from each level.
Inclusive vs exclusive day count
Algorithm:
Age in years — completed vs current year of life
1. Years = endYear − startYear. If end’s (month, day) is earlier in the year than start’s, subtract 1 (the last birthday hasn’t happened yet).
Pregnancy — weeks and months conventions
2. Months = endMonth − startMonth (mod 12 if negative), adjusting year if needed.
Legal age — varies by purpose and jurisdiction
3. Days = endDay − startDay. If negative, borrow from the previous month’s day count (and decrement months by 1).
Biological vs chronological age
Example: start 1990-03-15, end 2026-04-23 → Years = 36, Months = 1, Days = 8. (March 15 1990 → March 15 2026 = 36 years; + 1 month + 8 days to reach April 23 2026.)
Pet age conversions — beyond “dog years”
Leap year: divisible by 4, except century years unless divisible by 400. So 2000 was a leap year, 1900 was not, 2100 will not be.
Business-date arithmetic
Someone born February 29, 2000: when exactly do they turn 25? Legally (most jurisdictions) on March 1, 2025 (non-leap years). In leap years (2024, 2028), they turn on the 29th. Software that doesn’t handle this returns “March 1” or crashes on Feb 29 inputs.
Time zones — when the dates differ by zone
Day-count math across leap years: if spans include Feb 29, that day counts normally — ~365.25 days per year average.
Common calculation errors
Asked “how many days from March 15 to March 20”, two valid answers:
Run the numbers
Ambiguous in conversation; context determines convention. Legal and medical often prefer inclusive; financial/duration math prefers exclusive.