How this calculator works
The most recent birthday is found first, with a February 29 birthday clamped to February 28 in a common year. Whole months are then counted forward from that anniversary until the next month would pass the measurement date. What remains is the day count. This produces the familiar years, months and days answer and keeps it consistent with the exact total day count.
Formula
Let A be the birth date shifted forward by a whole number of years so that A is not after the measurement date. years = measurement year - birth year, reduced by one if that anniversary has not yet arrived. months = the largest m with A plus m months not after the measurement date. days = O(measurement) - O(A plus months). total_months = 12 x years + months.
Worked example
Born 1990-05-20 and measured on 2026-11-01: 36 years, 5 months and 12 days. That is 437 total months and 13314 total days. A leap-day birth on 2000-02-29 measured on 2023-02-28 is exactly 23 years, 0 months and 0 days.
Advertisement
Frequently asked questions
How is this different from age in completed years?
Completed years answer "how many birthdays have passed". Chronological age also gives the leftover months and days, which is what growth charts, development questionnaires and some clinical records ask for.
Why do the components not simply divide the total days?
Calendar months are 28 to 31 days long, so dividing total days cannot reproduce them. The page counts real calendar months and then uses days for the remainder.
What happens to a February 29 birthday?
In a common year the anniversary is taken as February 28. That matches the convention used by the age calculator and by most Canadian age rules, though a statute can choose March 1 for a specific purpose.
Is chronological age the same as biological age?
No. Chronological age is simply time since birth. Biological or developmental age compares a person with population norms and cannot be derived from two dates.