How to use the chronological age calculator
- 1Enter the Birth date and the Test date (today by default).
- 2Choose the Method: Borrow 30 days (hand method) as printed in many manuals, or Calendar months.
- 3Set Rounding if your manual requires it, and tick Also calculate adjusted age for prematurity if the child was born early.
- 4Read the age as years;months;days with the worked subtraction underneath, and press Copy result.
Birth 23 June 2018, tested 5 October 2026
Birth date 2018-06-23 · Test date 2026-10-05 · Borrow 30 days
8;3;12 (8 years, 3 months, 12 days) Total months: 99 Rounded at 16 days or more: 8;3
Days: 5 is less than 23, so 1 month is borrowed as 30 days: 35 − 23 = 12 days, and September (9) − June (6) = 3 months.
Birth date and test date
Chronological age (CA) is the exact time between a person's birth date and the date they are tested. Standardized tests in psychology, education, and speech and language therapy convert raw scores using norms tables split by age bands (for example 8;0 to 8;3), so a few days can move a child into a different band.
Age is written years;months;days, or years:months, so 8;3;12 means 8 years, 3 months and 12 days. Record the date of the first testing session unless the manual says otherwise for testing spread over several days.
Borrowing days and months: the method
Write the test date above the birth date as year, month and day, and subtract from right to left:
| Year | Month | Day | |
|---|---|---|---|
| Test date | 2026 | 10 → 9 | 5 → 35 |
| Birth date | 2018 | 6 | 23 |
| Age | 8 | 3 | 12 |
- If the test day is smaller than the birth day, borrow 1 month from the month column and add 30 days.
- If the test month is then smaller than the birth month, borrow 1 year and add 12 months.
Calendar months instead counts real month lengths. The two can differ by a day or two around month ends: from 31 January 2019 to 1 March 2026 is 7;1;0 by the 30-day method and 7;1;1 by the calendar method.
Rounding rules in test manuals
Manuals differ, which is why the calculator labels each option:
- Don't round: report years;months;days and use years;months to find the norms band. Many current tests work this way.
- Round up at 16 days or more or at 15 days or more: older manuals that report age in whole months. 8;3;12 stays 8;3; 8;11;20 becomes 9;0.
- Drop the days: never round up.
Use exactly the rule printed in your test's administration or scoring manual. Rounding the wrong way can shift a score by a whole norms band.
Adjusted (corrected) age for prematurity
For babies born before 37 weeks, development is often judged against adjusted age: age counted from the due date (40 weeks) rather than the birth date. Adjusted age = chronological age − weeks born early.
A baby born on 10 March 2025 at 32 weeks has a due date of 5 May 2025. On 20 January 2026 their chronological age is 0;10;10 and their adjusted age is 0;8;15. Adjustment is usually used until about 24 months chronological age; check whether your test's norms expect corrected age and up to what age.
Chronological vs developmental age
Chronological age is calendar time since birth, and it is what you enter into a norms table. Developmental age (or age-equivalent) is an output of testing: the age at which a typical child achieves a given raw score. A 9-year-old with an age-equivalent of 7;6 on a vocabulary test scored like a typical child of 7½.
Age-equivalents are coarse and easy to misread, so most publishers recommend reporting standard scores and percentiles, which are calculated from chronological age.
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