Age looks like simple subtraction until leap years, month lengths and the difference between calendar age and elapsed days get involved. This calculator handles all three, and this page explains why two age calculators can legitimately disagree by a day.
How age is counted
The standard method counts completed years, then completed months, then remaining days:
- Subtract the birth year from the current year.
- If the current month is before the birth month, subtract one year.
- If it is the same month but the day is earlier, subtract one year.
- Count remaining months and days from the last birthday.
Someone born on 15 August 1990, on 30 August 2026, is 36 years and 15 days old.
Where calculators disagree
Month lengths are unequal, so 'one month' has no fixed value. From 31 January, adding one month gives 28 February in most implementations and 3 March in others, depending on whether the result is clamped to the month end or allowed to overflow. This is why two tools can differ by a day or two on the months-and-days breakdown while agreeing exactly on total days elapsed.
Total elapsed days is unambiguous and is the figure to use when precision matters.
Leap years
A year is a leap year if divisible by 4, except centuries, unless divisible by 400. So 1900 was not a leap year and 2000 was. This three-part rule keeps the calendar aligned with the solar year of about 365.2422 days.
People born on 29 February have a birthday in only about one year in four. Legal systems handle this differently: many treat 1 March as the legal birthday in common years, some use 28 February, and the choice occasionally matters for eligibility thresholds.
Different counting conventions
| System | How it works |
|---|---|
| International | Age 0 at birth; increases on each birthday |
| Traditional East Asian | Age 1 at birth; increases at new year, not on the birthday |
| Insurance 'age nearest' | Rounds to the nearest birthday, so you can be rated a year older six months early |
South Korea moved to the international system for most official purposes in 2023, having previously used a traditional count that could make a person two years older than their international age.
Practical uses
- Days lived — the unambiguous total, useful for milestone calculations.
- Next birthday countdown — days remaining, accounting for leap years in between.
- Age on a specific date — for eligibility checks against a deadline rather than today.
- Age gap between two people — expressed in years, months and days.
Time zones
Age depends on today's date, which depends on where you are. Someone checking just after midnight in Sydney and someone checking the same moment in Los Angeles are on different calendar days. This calculator uses your device's local date, which is almost always what you want — but it means a result can differ by a day from one calculated elsewhere at the same instant.