Chronological Age Calculator
Chronological age calculator. Enter a date of birth and a test (or as-of) date, defaulting to today. The calculator returns chronological age in years, months, and days using standard borrowing (a borrowed month counts as 30 days, as most test manuals specify), plus total months and total days. Optional toggle to show the calendar-exact version that borrows the real length of the prior month. Runs fully in the browser.
How to use
- Enter the date of birth.
- Enter the test or as-of date, or leave it on today.
- Keep the 30-day month, which most test manuals use, or switch to calendar-exact.
- Read the age in years, months, and days, the total months, the total days, and the borrowing steps.
Formula
Write the test date above the birth date as year, month, and day, then subtract from right to left. If the days are short, borrow one month as 30 days, or as the real length of the previous month. If the months are short, borrow one year as 12 months. Total months is years × 12 plus the leftover months. Total days is the exact number of calendar days between the two dates, including leap days.
Example. Birth 2015-08-20 and test 2026-03-05 is 10 years, 6 months, 15 days with the 30-day method. The day column borrows 30, so 35 − 20 = 15 days and the month column becomes 2. The month column then borrows 12, so 14 − 8 = 6 months and 2025 − 2015 = 10 years.
Example. Birth 2020-01-31 and test 2026-01-31 is 6 years, 0 months, 0 days.
Example. Birth 2018-12-15 and test 2026-11-04 is 7 years, 10 months, 19 days.
Rounding is up to the manual
This page does not round the age. Some tests round up when the days are 15 or more, and others ignore the days. Use the scoring rules for the test you are giving.
Frequently asked questions
What is chronological age?
Chronological age is the exact time that has passed since a person was born, written in years, months and days. It is used to score developmental and educational tests.
How do you calculate chronological age by hand?
Write the test date above the birth date as year, month, day and subtract each column from right to left. If the days are short, borrow one month as 30 days; if the months are short, borrow one year as 12 months.
Why does a borrowed month count as 30 days?
Most test publishers use a fixed 30-day month so every examiner gets the same answer. The calendar-exact option borrows the real number of days in the previous month instead.
Should I round chronological age?
It depends on the test manual. Some tests round up when the days are 15 or more, while others ignore the days entirely, so check the scoring rules for your test.
What is the difference between chronological age and adjusted age?
Chronological age counts from the actual birth date. Adjusted age, used for premature babies, subtracts the weeks the baby was born early.