Standardised tests such as reading, IQ and developmental assessments score a child against peers of the same chronological age, so the age has to be exact. Many test manuals specify borrowing 30 days for a month; others use the actual length of the previous month. Some round days of 15 or more up to the next month.
How to use the Chronological Age Calculator
- Date of birth
- Test date
- Borrow for a month
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the date of birth and the test date, and choose the borrowing method your test manual uses.
Tips
- Check whether the end date should be included – event durations usually include it, elapsed time does not.
- Months have 28–31 days, so “one month” is counted by calendar date, not as 30 days.
- Use 24-hour time or set AM/PM carefully when entering clock times.
- Time zones and daylight-saving changes are not applied unless the calculator says so.
Common mistakes to avoid
- Counting a 30-day month when a calendar month is meant (or the reverse).
- Forgetting leap years when counting days across February.
- Mixing decimal hours (7.5 h) with clock time (7:30).
Inputs explained
| Field | What to enter |
|---|---|
| Date of birth | A date |
| Test date | A date |
| Borrow for a month | Choose one: 30 days (most test manuals), Actual days in the previous month |
What each result means
- Age in total months = [Years]×12 + [Months]
- Age in years (decimal) = ([Test date] – [Date of birth])÷365.2425
Formula
Subtract the birth date from the test date (year, month, day)If days are negative, borrow one month (30 days, or the days in the previous month)If months are negative, borrow one year (12 months)Worked example
Inputs — Date of birth: 2017-09-24; Test date: 2026-10-25; Borrow for a month: 30 days (most test manuals).
Results — Days = 1; Months = 1; Years = 9; Age in total months = 109; Age in months (rounded, 15+ days up) = 109; Age in years (decimal) = 9.08.
Frequently asked questions
How do I calculate chronological age?
Write the test date and birth date as year–month–day and subtract, borrowing 30 days or 12 months when needed.
Do I round chronological age?
Check your test manual – many do not round; some round 15 days or more up to the next month.
What is the difference between chronological and corrected age?
Corrected age subtracts the weeks a premature baby was born early.
What formula does the Chronological Age Calculator use?
Subtract the birth date from the test date (year, month, day).