Testing over several sessions? The WIAT-III, for one, uses the first session’s date.
Borrowing methodMost test manuals count every borrowed month as 30 days.
40 minus the gestational age at birth. Leave 0 for a full-term birth.
Chronological age7;9;18
In words7 years, 9 months, 18 days
Exact age (Y;M;D)7;9;18
Age in whole months93
Age in days2,849

Borrowing method: when the test day is smaller than the birth day, 1 month is borrowed as 30 days; when the test month is smaller, 1 year is borrowed as 12 months.

Worked subtraction
StepYearMonthDay
Test date2026109
After borrowing20252139
− Date of birth20181221
= Chronological age7918

Chronological Age Calculator · free calculator by CalcWorthy