Guide · 8 min read
How to Calculate Chronological Age (Step by Step)
Learn how to calculate chronological age by hand from a birth date to the current date or a test date, with two methods, worked examples, practice questions and a ready-made Excel formula.
By Sandeep · Updated November 2026
The Chronological Age Formula
Chronological age is the time from the date of birth to a chosen date, usually the day of a test. The formula is simple subtraction:
The only tricky part is borrowing, because months have 28, 29, 30 or 31 days. That is why many people prefer a chronological age calculator, but knowing the method helps you check any result.
How to Calculate Chronological Age by Hand (Manually)
- Write both dates in columns. Put the test date on top and the date of birth below it, as year, month and day.
- Subtract the days. If the test day is smaller than the birth day, borrow 1 month: take 1 from the test month and add the number of days in the month before the test month to the test day.
- Subtract the months. If the test month is now smaller than the birth month, borrow 1 year: take 1 from the test year and add 12 to the test month.
- Subtract the years. Write the answer as years;months;days, for example 7;2;16.
This is the same method printed on most standardized test record forms used by speech-language pathologists, psychologists and occupational therapists. Once you know it, you can calculate age manually for any two dates, without an app.
The Calendar Method: Counting Forward
Some people find it easier to count forward on a calendar instead of subtracting. This calendar method gives the same answer:
- Count full years from the birth date to the last birthday before the test date.
- Count full months from that birthday, stopping before you pass the test date.
- Count the remaining days up to the test date.
Example: born 25 January 2019, tested 10 April 2026. The last birthday was 25 January 2026, so that is 7 years. Two more months reach 25 March 2026. From 25 March to 10 April is 16 days. Answer: 7;2;16, the same as the subtraction method in Example 2 below.
Pro tip: use the calendar method to double-check a result, and the subtraction method when you need to show your working on a record form or legal documents.
4 Worked Examples
Example 1: No borrowing
| Year | Month | Day | |
|---|---|---|---|
| Test date | 2026 | 9 | 20 |
| Date of birth | 2018 | 5 | 12 |
| Chronological age | 8 | 4 | 8 |
Every top number is larger, so just subtract. Answer: 8;4;8.
Example 2: Borrowing a month
| Year | Month | Day | |
|---|---|---|---|
| Test date | 2026 | 4 | 10 |
| After borrowing | 2026 | 3 | 41 |
| Date of birth | 2019 | 1 | 25 |
| Chronological age | 7 | 2 | 16 |
10 is smaller than 25, so borrow March (31 days): 10 + 31 = 41. Answer: 7;2;16.
Example 3: Borrowing a month and a year
| Year | Month | Day | |
|---|---|---|---|
| Test date | 2026 | 3 | 8 |
| After borrowing | 2025 | 14 | 36 |
| Date of birth | 2017 | 11 | 20 |
| Chronological age | 8 | 3 | 16 |
Borrow February 2026 (28 days): 8 + 28 = 36, month becomes 2. Then 2 is smaller than 11, so borrow a year: 2 + 12 = 14 and 2026 becomes 2025. Answer: 8;3;16.
Example 4: Leap year
| Year | Month | Day | |
|---|---|---|---|
| Test date | 2024 | 3 | 5 |
| After borrowing | 2024 | 2 | 34 |
| Date of birth | 2020 | 1 | 30 |
| Chronological age | 4 | 1 | 4 |
2024 is a leap year, so February has 29 days: 5 + 29 = 34. Answer: 4;1;4. Using 28 days here would give a wrong answer of 4;1;3.
How to Calculate Chronological Age in Excel or Google Sheets
Put the date of birth in cell A2 and the test date in B2, then use the DATEDIF function:
This returns the age in Y;M;D format and works in both Excel and Google Sheets. For age in total months, use =DATEDIF(A2,B2,"M").
Caution: Microsoft warns that the “MD” option can give wrong day counts in some cases. Always double-check the days for test scoring with the hand method or our online calculator.
Common Mistakes When Calculating Chronological Age
- Treating every month as 30 days. Always borrow the real length of the month before the test month.
- Borrowing the wrong month. For a test on 10 April, borrow March, not April.
- Forgetting leap years. February has 29 days in 2024, 2028 and 2032.
- Rounding when the manual does not ask for it. Check the rules in our age rounding rules for testing guide.
- Mixing up the birth month and birth year. Write dates as year, month, day before you subtract, especially with US (month/day) and UK (day/month) formats.
- Using the report date instead of the test date. Age must be calculated on the day testing took place.
- Not correcting for prematurity. For babies born early, also find the age with a corrected age calculator.
Practice Questions
Try these, then open each answer to check your work. More are in our chronological age examples.
1. Born 3 February 2020, tested 15 June 2026
No borrowing needed: 2026 − 2020 = 6, 6 − 2 = 4, 15 − 3 = 12. Answer: 6;4;12.
2. Born 22 August 2016, tested 9 January 2026
Borrow December 2025 (31 days): 9 + 31 = 40, month 0. Borrow a year: 0 + 12 = 12, year 2025. Then 2025 − 2016 = 9, 12 − 8 = 4, 40 − 22 = 18. Answer: 9;4;18.
3. Born 31 December 2021, tested 30 November 2025
Borrow October (31 days): 30 + 31 = 61, month 10. Borrow a year: 10 + 12 = 22, year 2024. Then 2024 − 2021 = 3, 22 − 12 = 10, 61 − 31 = 30. Answer: 3;10;30.
How to Calculate Chronological Age: FAQs
How do you find chronological age?
Subtract the date of birth from the date you need the age for, column by column: days, months, then years. Borrow a month or a year when a column is too small. Or enter both dates in a chronological age calculator for an instant answer.
How do you write chronological age?
Write it as years;months;days, for example 7;2;16 for 7 years, 2 months and 16 days. Some forms use a dash (7-2-16) or only years and months (7;2).
Do you count the day of birth when calculating age?
No. The birth date is the starting point, not a full day lived. A baby tested the day after birth is 0;0;1.
How do you calculate chronological age in months?
Multiply the years by 12 and add the months. A child who is 4;7;10 is 55 months and 10 days old. Our age in months calculator does this for you.
How do occupational therapists calculate chronological age?
Occupational therapists use the same borrowing method on their assessment record forms. For children born prematurely and under 2, many OT assessments also ask for corrected age.
Why does my answer differ from an online calculator by one day?
Usually because one method treats every month as 30 days or skips a leap day. Borrow the real number of days in the month before the test month and the results will match.