How to Calculate Chronological Age by Hand
The column method takes under a minute once you know the two borrowing rules. Use it to check the chronological age calculator, or to teach a student clinician how the number on the protocol is produced.
The column method in four steps
| Year | Month | Day | |
|---|---|---|---|
| Test date | 2026 | 3 | 15 |
| Date of birth | 2019 | 8 | 22 |
| Age | 6 | 6 | 21 |
- Days. 15 − 22 cannot be done, so borrow one month from the month column. The month before the test month is February 2026, which has 28 days: 15 + 28 = 43, and 43 − 22 = 21. The month column is now 2 instead of 3.
- Months. 2 − 8 cannot be done, so borrow one year: 2 + 12 = 14, and 14 − 8 = 6. The year column is now 2025.
- Years. 2025 − 2019 = 6.
- Write it down. 6 years, 6 months, 21 days, or 6;06;21 on the record form.
The two borrowing rules
Rule 1, days. When you borrow a month to rescue the days column, you add the number of days in the month before the test month, because that is the month the borrowed days come from. If the test was in May, add 30; in August, add 31; in March, add 28 (or 29 in a leap year). Some test manuals simplify this to "always add 30"; both conventions are available on the calculator and usually give the same answer.
Rule 2, months. When you borrow a year to rescue the months column, you always add 12.
Always work right to left, days first, because borrowing in the days column changes the months column before you subtract it.
Worked example 2: no borrowing needed
Test date October 20, 2026; born April 5, 2020. Days: 20 − 5 = 15. Months: 10 − 4 = 6. Years: 2026 − 2020 = 6. Age 6;06;15. When every column subtracts cleanly there is nothing to borrow.
Worked example 3: leap-year birthday
Test date March 1, 2025; born February 29, 2020. Days: 1 − 29 is negative, so borrow a month and add the days of February 2025 (28): 1 + 28 − 29 = 0. Months: (3 − 1) − 2 = 0. Years: 2025 − 2020 = 5. Age 5;00;00: on March 1, 2025 the child is exactly five, which is how a February 29 birthday is normally treated in a non-leap year.
Writing the result: Y;M;D
Record forms use semicolons between years, months and days: 6;06;21. Keep two digits for months and days so columns line up and a 6;6 is never misread as 66. If the form only has space for years and months, drop the days without rounding: 6;06. If the manual wants total months, multiply the years by 12 and add the months: 6 × 12 + 6 = 78 months.
Common mistakes
- Subtracting left to right (years first) and forgetting that borrowing later changes an earlier column.
- Adding the days of the birth month, or of the test month, instead of the month before the test month.
- Rounding days up to the next month when the manual does not ask for it.
- Using today's date instead of the date the test was actually administered.
- Typing a date in the wrong order (day/month versus month/day) when entering it into software.
Practice
Try these by hand, then confirm them with the calculator: (a) test July 4, 2026, born November 30, 2021; (b) test January 10, 2026, born January 11, 2016; (c) test December 31, 2026, born December 31, 2026. Answers: 4;07;04 · 9;11;30 · 0;00;00.