The same algorithm we use everywhere
This tool, the Age Calculator, and the Date Duration Calculator all share one calendar-anchor algorithm: borrow days from the previous month if needed, then borrow months from the year. The result is the intuitive answer that matches Java’sPeriod.between, Python’s dateutil, and spreadsheet DATEDIF.
Why it matters at the boundaries
The naïve approach — subtract milliseconds and divide — drifts wildly across leap years and variable-length months. Two siblings born exactly one calendar year apart should read “1 year, 0 months, 0 days” — not “0 y 11 mo 30 d”. The anchor algorithm produces the right answer for both leap-year and non-leap-year crossovers.
Common uses
- Sibling age gaps for school registration, pediatric well-visits, family planning.
- Partner age differences (out of curiosity or for legal/insurance forms).
- Twin separation (down to days/weeks where time-of-birth doesn’t matter for your purpose).
- Comparing dog or cat age (use our Dog Age and Cat Age calculators in parallel).