Birth Date Year Calculator
Birth Year Calculator works out Age in years and Extra months from Age at this date, Age in years and Extra months of age. The default figures on this page give date of birth of 2000-06-15; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
8 steps
- Working backwards from 2025-06-15 for an age of 25 years, 0 months, 0 days.
- Subtract the days, then the months, then the years: 2025-06-15 - 0 day(s) - 0 month(s) - 25 year(s) = 2000-06-15.
- A whole-year age does not fix the day: anyone born from 1999-06-16 through 2000-06-15 is 25 on 2025-06-15.
- Born 2000-06-15 (a Thursday); measuring on 2025-06-15.
- Whole years first: 2000-06-15 + 25 years lands on or before 2025-06-15.
- Then whole months and days: + 0 month(s) + 0 day(s) reaches 2025-06-15.
- Elapsed days = 9,131, i.e. 1304 weeks and 3 day(s).
- Next birthday 2026-06-15 (Monday), 365 day(s) away; turning 26.
The same age in every unittable
Its 9 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
5 sources
What the Birth Year Calculator calculates
Age in completed years, months and days on any date, with the total days lived, the weekday of birth, the next birthday and the gap between two birth dates; it also runs backwards to recover a date of birth from an age, breaks an age down into every time unit, and corrects an infant age for prematurity. Age counts completed calendar months and days, so a 29 February birth attains the next year of age on 1 March in common years.
How to use this Birth Year Calculator
Enter Age at this date, Age in years, Extra months of age and Extra days of age. Age in years updates as you type. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Age in years, Extra months and Extra days.
The method follows ISO 8601-1:2019, Date and time - Representations for information interchange, Howard Hinnant, chrono-Compatible Low-Level Date Algorithms, US Social Security Administration, Program Operations Manual System GN 00302.400 (age computation), Squires J. and Bricker D., Ages & Stages Questionnaires, Third Edition (ASQ-3) User’s Guide, Brookes Publishing and American College of Obstetricians and Gynecologists, Committee Opinion 700: Methods for Estimating the Due Date.
What you can compare
Change one input at a time to see how it affects Age in years, Extra months and Extra days. The inputs you can vary are Age at this date, Age in years, Extra months of age and Extra days of age.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How do I calculate my birth year?
Age in completed years, months and days on any date, with the total days lived, the weekday of birth, the next birthday and the gap between two birth dates. Open “How this is worked out” beside the result to see each step for your own figures.
What is the date of birth year?
On the figures this page opens with — age in years 25 — the date of birth is 2000-06-15. No single number fits everyone: change any figure above and the answer updates as you type, with each step of the arithmetic beside it.
How do I find someone's birth year?
It needs Age at this date, Age in years, Extra months of age and Extra days of age. Nothing else is required, and the figures stay in your browser.
What information do I need for the Birth Year Calculator?
You need Age at this date, Age in years, Extra months of age and Extra days of age. The page opens with default figures that give date of birth of 2000-06-15, so you can see the scale before entering your own.
What does this Birth Year Calculator show?
It shows Age in years, Extra months, Extra days, Age, Chronological age (years;months), Age in decimal years, Age in months and Age in weeks, with the working and any warnings for your entries.
Can I check how the Birth Year Calculator result was calculated?
Yes. The “How this is worked out” section shows the calculation steps, and the sources section identifies the references used for the method.