How age is calculated

Count completed years first

A person turns one year older only when the anniversary of the birth date is reached. Before that anniversary, subtract one from the simple year difference.

Then borrow real calendar months

If the as-of day is earlier in the month than the birth day, borrow the actual number of days in the preceding month. That is why month length changes the days remainder.

Leap years affect elapsed days

Leap days change the total-day interval but do not make a birthday arrive early. Policies for a February 29 birthday can depend on jurisdiction, so the calculator reports neutral calendar math.

Useful distinction: “25 years old” and “9,131 days elapsed” answer different questions.

A worked example, step by step

Take a birth date of 17 November 1990 and an as-of date of 4 March 2026. Count years, then months, then days, borrowing only when a column would go negative.

Years: 2026 − 1990 = 36, but 4 March comes before the 17 November anniversary, so only 35 years are complete.

Months: from 17 November 2025 to 17 February 2026 is three whole months. 17 March has not arrived, so the count stops at three.

Days: February 2026 has 28 days, so 17 February to 28 February is 11 days, and four more reach 4 March: 15 days.

Result: 35 years, 3 months, 15 days. Had the February in between been a leap-year February with 29 days, the remainder would have been 16.

Which answer a form is asking for

Most forms that ask for age want completed years on a specific date, and that date matters as much as the birthday.

Eligibility rules are usually written as an age reached on or by a particular date — the first day of a school year, a policy start date, an election day. Enter that date as the as-of date rather than today, and read the completed years.

Interval questions — how long ago something happened, how old a warranty or an account is — are better answered in years, months, and days, or in total days, because rounding to whole years can hide almost a full year.

Total days is the right unit when the interval feeds another calculation, such as a daily rate or an average, because it does not depend on how long each month happens to be.