The setup
Write both dates in year – month – day order, test date on top:
| Year | Month | Day | |
|---|---|---|---|
| Test date | 2026 | 07 | 08 |
| Date of birth | 2018 | 03 | 02 |
Example 1 — no borrowing needed
- Days: 08 − 02 = 6 days
- Months: 07 − 03 = 4 months
- Years: 2026 − 2018 = 8 years
Chronological age: 8 years, 4 months, 6 days → 8;4.
Example 2 — with borrowing (where mistakes happen)
Test date 2026-02-05, date of birth 2015-06-15.
- Days: 05 − 15 is negative, so borrow one month. The month before February 2026 is January, which has 31 days. So days become 05 + 31 = 36, and 36 − 15 = 21 days. The month column drops from 02 to 01.
- Months: 01 − 06 is negative, so borrow 12 from the year: 13 − 6 = 7 months. The year column drops from 2026 to 2025.
- Years: 2025 − 2015 = 10 years.
Chronological age: 10 years, 7 months, 21 days → 10;7.
The three most common mistakes
- Borrowing a flat 30 days. The correct borrow is the real length of the month before the test date — 28, 29, 30 or 31. Using 30 every time produces off-by-one and off-by-two errors.
- Forgetting the borrowed year. After borrowing 12 months, the year column must drop by 1. Skipping this inflates the age by a full year.
- Rounding days into months. Some assessment manuals explicitly say not to round up. 6 years, 5 months, 29 days is 6;5 — not 6;6 — unless your manual says otherwise.
Why precision matters in assessments
Standardized tests such as CELF-5, PPVT-5 and EVT-3 pick their scoring norms from the child's exact age on the test date. A calculation error of even one month can select the wrong norm table, produce misleading standard scores, and in the worst case contribute to a misdiagnosis. That is why examiners are trained to compute — and double-check — chronological age before scoring anything.
The fastest way to double-check your manual work is to run the same dates through our chronological age calculator, which applies the exact borrowing method described above with true month lengths and leap years.