Method used in test manuals
Test date − Birth date, column by column (year, month, day)
Borrow 30 days when days are negative; borrow 12 months when months are negative
Borrow 30 days when days are negative; borrow 12 months when months are negative
Example: Born May 20, 2018, tested September 10, 2026. Days: 10 − 20 is negative, so borrow 30 days: 40 − 20 = 20 days, and August becomes month 8. Months: 8 − 5 = 3. Years: 2026 − 2018 = 8. Chronological age 8;3.20 (the calendar method gives 8 years, 3 months, 21 days).
Tips for examiners
- Double-check the test date on the protocol matches the session date.
- For premature infants, some developmental tests use a corrected age.
Need someone’s everyday age instead? Use the age calculator.
Frequently asked questions
How do you calculate chronological age?
Subtract the birth date from the test date: year from year, month from month, day from day. If the days are negative, borrow a month (30 days); if months are negative, borrow a year (12 months).
Why do test manuals borrow 30 days?
It keeps hand calculations consistent. The calendar method borrows the actual number of days in the previous month, so the result can differ by a day or two.
Should I round up the days?
Most tests (for example many Pearson assessments) do not round — you use years and months and drop the days. Some tests round 15 or more days up. Always follow the manual for the test you are scoring.
How is chronological age written?
Commonly as years;months (for example 8;3) or years;months.days (8;3.20).