How to Calculate Age and Date Differences
Learn exactly how age is computed from two dates, how leap years and birthdays affect the result, and a hand method for any date difference. Free calculator inside.
How old am I, exactly? It sounds like a question with an obvious answer, but computing age from dates — in years, months, and days — trips up more people than you would expect. Leap years, month lengths, and whether a birthday has passed this year all change the result.
The same arithmetic answers many everyday questions: how long someone has worked at a company, how many days until a deadline, or how many weeks along a pregnancy is. This guide shows how the calculation works, how to do it by hand, and where the edge cases hide — or skip the arithmetic with our free Age Calculator.
How Age Is Computed, Step by Step
Computing age means answering three questions in order: how many full years, then how many full months, then how many days. Start with the years: subtract the birth year from the current year, then subtract one more year if the birthday has not happened yet this year.
Next the months: count the whole months between the birthday month and the current month, adjusting if the birthday day has not arrived yet. Then count the remaining days. Someone born on 14 March 1990, measured on 8 October 2026, is 36 years, 6 months, and 24 days old.
The fiddly part is that months have different lengths, so one month after 31 January is not a fixed number of days — most calculators resolve it by counting calendar months rather than 30-day blocks. That is also why doing it by hand feels harder than it should.
Leap Years and the February 29 Problem
A year is a leap year if it is divisible by 4, except century years, which must also be divisible by 400 — so 2024 was a leap year, 1900 was not, and 2000 was. The extra day keeps the calendar aligned with the Earth's orbit.
People born on February 29 face a small annual puzzle: in common years, is their birthday February 28 or March 1? Most legal systems pick one by convention, and most age calculators treat March 1 as the birthday in non-leap years, though some use February 28 — which is why two tools can disagree by a day.
For ordinary date differences, leap days simply count as days like any other. A span that crosses February 29 of a leap year contains one more day than the same span in a common year, and careful calculators account for this automatically.
The Hand Method: Borrowing Like Subtraction
To find the difference between two dates by hand, write the later date above the earlier one as year-month-day and subtract column by column, borrowing exactly as in ordinary subtraction. If the day column goes negative, borrow the number of days in the previous month; if the month column goes negative, borrow 12 months from the year.
Example: from 14 March 1990 to 8 October 2026. Days: 8 minus 14 will not work, so borrow September's 30 days: 38 minus 14 = 24 days. Months: October is month 10, but we borrowed one, so 9 minus 3 = 6 months. Years: 2026 minus 1990 = 36 years. Result: 36 years, 6 months, 24 days.
The only step that needs care is knowing how many days to borrow, which depends on the month you borrow from — 28, 29, 30, or 31. Get that right and the method never fails; get it wrong and the day count slips by one or two.
Beyond Birthdays: Everyday Uses
The same arithmetic measures job tenure (five years and three months of experience), project timelines, and rental periods. Employers, landlords, and contracts all run on exact date differences, and being able to compute them removes a surprising amount of guesswork.
Pregnancy is counted in weeks from the first day of the last menstrual period — 40 weeks being the standard full term — which is just a date difference expressed in weeks. Deadlines work the same way: the days remaining until a date, counted exclusively of today, tell you exactly how much runway is left.
Anniversaries, warranty periods, subscription renewals, and visa durations are all date differences in disguise. Whenever precision matters — a contract, a medical schedule, a legal deadline — compute the exact span rather than rounding to about six months. Our free Age Calculator handles any two dates, not just birthdays.
Mistakes That Throw Off the Count
The most common error is the off-by-one: counting both the start and end dates when you should count only one of them. If an event runs from Monday to Friday, that is 4 days of elapsed time but 5 calendar days touched — decide which one you mean before you start.
The second is treating every month as 30 days. Over a few months the error is small, but across years of tenure or interest calculations it compounds into something meaningful. Always borrow the actual month length.
Finally, remember that time of day and time zone rarely matter for calendar-age math but matter enormously for deadlines: due October 10 in one time zone can be October 11 in another. For legal and travel purposes, pin down the exact moment, not just the date.