To calculate age from a date of birth, subtract the date of birth from the later date one column at a time: days first, then months, then years. When a column comes out negative, you borrow from the column to its left.
The 3-column method
Write the later date above the date of birth as year, month and day.
- Days. Subtract the birth day from the later day. If the result is negative, borrow 1 month: add the number of days in the month before the later date’s month, and take 1 away from the months column.
- Months. Subtract the birth month. If the result is negative, borrow 1 year: add 12 months and take 1 away from the years column.
- Years. Subtract the birth year. The 3 results are the age in years, months and days.
A worked example
Someone born on 15 November 2006, on 10 March 2026:
| Year | Month | Day | |
|---|---|---|---|
| Later date | 2026 | 3 | 10 |
| Date of birth | 2006 | 11 | 15 |
- Days: 10 − 15 is negative. February 2026 has 28 days, so 10 + 28 = 38, and 38 − 15 = 23 days. The month column drops from 3 to 2.
- Months: 2 − 11 is negative, so borrow a year: 2 + 12 = 14, and 14 − 11 = 3 months. The year column drops from 2026 to 2025.
- Years: 2025 − 2006 = 19 years.
The age is 19 years, 3 months and 23 days.
Totals in days, weeks and hours
For a total in days, count every calendar day between the 2 dates, including each February 29. The example above spans 7,055 days. Divide by 7 for weeks (1,007 weeks and 6 days) and multiply by 24 for hours (169,320 hours).
Let the calculator do it
The age calculator runs the same 3-column subtraction with real month lengths and leap years, and shows the totals at once. For score sheets that need the Y;M;D format, use the Chronological Age Calculator.
