Age Difference Calculator
Age Difference Calculator works out Age in years and Extra months from Date of birth, Age at this date and Second date of birth (optional). The default figures on this page give age gap in years of 5 years; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
6 steps
- Born 1985-03-10 (a Sunday); measuring on 2024-01-01.
- Whole years first: 1985-03-10 + 38 years lands on or before 2024-01-01.
- Then whole months and days: + 9 month(s) + 22 day(s) reaches 2024-01-01.
- Elapsed days = 14,176, i.e. 2025 weeks and 1 day(s).
- Next birthday 2024-03-10 (Sunday), 69 day(s) away; turning 39.
- Age gap: 1985-03-10 to 1990-07-22 = 5 years, 4 months, 12 days (1960 days).
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 Age Difference 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 Age Difference Calculator
Enter Date of birth, Age at this date and Second date of birth (optional). 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 Date of birth, Age at this date and Second date of birth (optional).
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
What information do I need for the Age Difference Calculator?
You need Date of birth, Age at this date and Second date of birth (optional). The page opens with default figures that give age gap in years of 5 years, so you can see the scale before entering your own.
What does this Age Difference 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 Age Difference 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.