Chronological age calculator for testing

How to use the chronological age calculator

1. Check the test date. It starts as today. If you tested on another day, change it.
2. Enter the date of birth. The age shows straight away as years;months;days, with the working laid out the way record forms set it out.
3. Pick the borrowing rule your manual uses. 30 days is selected to start with. Choose That month's real days only if your manual says so.
4. Leave rounding off unless your manual tells you to round, and then pick its cut-off.

You also get the years;months form and the total months. Copy result gives you all of it as text for a report.

Chronological age meaning

Chronological age is the time a person has been alive, counted from the date of birth to a given date. For testing, that date is the day the test was given, and the age is written in years, months and days, like 7;4;17 for 7 years, 4 months and 17 days. Speech-language pathologists, school psychologists and teachers use it to pick the right age band in a test's norm tables, and to decide which tasks or start points apply.

How to calculate chronological age by hand

Write the test date above the date of birth, as year, month and day, and subtract one column at a time, starting with the days. Pearson's WIAT-III examiner's manual, as quoted on Pearson's support site, puts it like this:

"Subtract the birth date from the test date. For these calculations, all months are assumed to have 30 days. Ages are not rounded up to the next month or year."Pearson Assessment Support, WIAT-III test date age calculation (Examiner's Manual pp. 21 to 22)

The Vineland-4 Pilot Manual on pearsonassessments.com says the same: "all months are assumed to have 30 days". Here is a child tested on October 8, 2026 who was born on May 21, 2019:

1. Days: 8 is less than 21, so borrow one month as 30 days: 8 + 30 = 38, and the 10 months become 9. 38 minus 21 = 17.
2. Months: 9 minus 5 = 4. If the test month had been smaller than the birth month, you would borrow one year as 12 months and take 1 off the year.
3. Years: 2026 minus 2019 = 7.

The chronological age is 7;4;17, or 7;4 in years and months, which is 88 months in total.

Borrowing 30 days or the real month, and rounding

Borrowing 30 days is the rule printed in the Pearson manuals above, and the federal Maternal and Child Health Bureau's training module on the CDC growth charts teaches the same steps: "Add 30 days to the number of days in the date of the measure" and "Add 12 to the number of months." Some calculators and programs borrow the real length of the month before the test month instead (28, 29, 30 or 31 days), so results can differ by a day or two. They only differ when that month is not 30 days long:

In both, the months need a borrowed year too: 2 is less than 7, so 2 + 12 = 14, 14 minus 7 = 7 months, and 2025 minus 2018 = 7 years.

Rounding: Pearson's manuals say ages are not rounded up, and the Vineland-4 Pilot Manual gives the example that 15 years, 11 months, 29 days "is not rounded up" to 16. So the calculator does not round unless you tick the box. If your manual does round, it gives its own cut-off; pick 15 or 16 days to match. Follow your test manual: its rule always beats any calculator, this one included.

Testing over two sessions: the WIAT-III manual says to "use only the first testing date in the calculation." Other manuals may say otherwise, so check yours.

Birthdays on the 29th, 30th or 31st

With 30-day borrowing, every month is treated alike. With real-month borrowing, a birth day that a month does not have (the 31st in a 30-day month, or February 29 in a common year) is counted as that month's last day, the same rule this site's other age calculators use. Born January 31 and tested April 30, the child counts as a whole number of months old that day. Spreadsheets' DATEDIF function and some other calculators can treat these dates differently.

Chronological age in Excel or Google Sheets

To copy the 30-day hand method in a spreadsheet, put the date of birth in A2 and the test date in B2:

• Days: =IF(DAY(B2)>=DAY(A2),DAY(B2)-DAY(A2),DAY(B2)+30-DAY(A2))
• Total months, in C2: =(YEAR(B2)-YEAR(A2))*12+MONTH(B2)-MONTH(A2)-IF(DAY(B2)<DAY(A2),1,0)
• Years: =INT(C2/12) and months: =MOD(C2,12)

For March 10, 2026 and July 25, 2018 that gives 15 days and 91 months, so 7;7;15. =DATEDIF(A2,B2,"Y") and =DATEDIF(A2,B2,"YM") give the whole years and the months left over by the calendar, but be careful with days: Microsoft says "We don't recommend using the "MD" argument, as there are known limitations with it."

Chronological age vs developmental age (and biological age)

Chronological age only counts time since birth. Developmental age describes the level a child is working at, usually given as the age at which children typically reach that level, from a test or observation. The two can differ, and manuals use both. For example, the Vineland-4 Pilot Manual tells examiners to use the start point for the child's chronological age, but for a child who "may be functioning below chronological age level" to begin at their developmental age instead.

Chronological age vs biological age: biological age is a different idea again, an estimate of how old a body seems from health measures. A date calculator cannot work it out.

For a baby or toddler who was born early, the age counted from the due date may be used instead of the age from birth. The corrected age calculator works that out.

Questions and answers

Sources

Checked on 8 October 2026. If a rule has changed, please tell us.

• Pearson Assessment Support: WIAT-III test date age calculation
• Pearson: Vineland-4 Pilot Manual (PDF), age calculation and start points
• Maternal and Child Health Bureau training module: Using the CDC growth charts, plotting measurements (calculating age)
• Microsoft Support: DATEDIF function
• Google Docs Editors Help: DATEDIF