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How to Calculate Age from Dates
Full years, then leftover months and days — calendar age, not a health score
Calendar age is the elapsed time from a birth date to an as-of date, written as completed years, then remaining months, then remaining days. This tutorial walks through that breakdown and a two-date playground. Age here is civil arithmetic, not a medical assessment, not a diagnosis, and not biological age.
Written by the My Calculator Stack editorial team. About our methods
This is calendar age from two dates, not medical advice and not a diagnosis. It does not measure health, development, or legal capacity. Time-zone and local-midnight conventions can shift a result by a day around midnight. Talk to a qualified clinician if you need age for care, dosing, or screening cutoffs.
1.What calendar age is
Calendar age counts how much time has passed from birth to a chosen as-of date. The usual civil breakdown is completed whole years, then leftover whole months, then leftover days. It is not biological age, not a fitness score, and not a diagnosis. The as-of date is often today, but any later date works — for example, age on a future birthday or on a form’s filing day.
2.The formula
Let birth be t₀ and as-of be t. Years y are the largest integer such that the anniversary t₀ + y years is on or before t (if the anniversary month/day has not been reached this year, y is one less than the year difference). From that anniversary, months m are the leftover whole months, then days d are the leftover calendar days. If as-of is before birth, age is undefined on this page. End-of-month birthdays follow ordinary civil calendar rules (the next month clamps to the last valid day).
\[ \mathrm{age} = (y\ \mathrm{years},\ m\ \mathrm{months},\ d\ \mathrm{days}) \]
3.Worked example
Take birth 1 January 2000 and as-of 14 August 2026 — a fixed pair so the arithmetic does not depend on “today.” The 2026 birthday (1 January) has already passed, so completed years = 26. From 1 January 2026 to 14 August 2026 there are 7 whole months (January through July) and then 13 leftover days (1 August to 14 August). Age = 26 years, 7 months, 13 days. The age calculator uses the same year-then-month-then-day breakdown.
\[ 2000\text{-}01\text{-}01 \to 2026\text{-}08\text{-}14 = 26\mathrm{y}\ 7\mathrm{m}\ 13\mathrm{d} \]
4.Two-date playground
Pick a birth date and an as-of date to see years, months, and days update along a simple span. This is a teaching widget — no legal-age flags, no full calculator chrome. Open the age calculator when you want the standalone tool with the same civil breakdown.
Inline playground
Calendar age
26 years 7 months 13 days
2000-01-01 → 2026-08-14
Calendar arithmetic only — not medical advice, not a diagnosis, not biological age.
5.Open the full calculator
The age calculator uses the same completed-years-then-months-then-days arithmetic, with birth and as-of dates and How it works on one page. Browse other date-time calculators if you need days between dates or a date offset next. Calendar age remains arithmetic, not a health score.
Try it yourself
Open the related calculator and put these formulas to work.