Why age is not just days divided by 365
Calendar age counts whole years, then whole months, then leftover days — it does not divide the total by 365.25. This matters at month boundaries: someone born on 31 January is one month old on 28 February in a common year, because there is no 31st.
The total-days figure is exact and accounts for every leap day in between, which is why it rarely equals the years figure times 365.
Leap-day birthdays
People born on 29 February have a birthday only once every four years. Legally, most jurisdictions treat them as ageing on 1 March in common years, though some use 28 February. The distinction occasionally matters for coming-of-age thresholds.
The total-days figure above is unaffected by any of this — it counts actual elapsed days, including every leap day that fell in the period.
How age is reckoned elsewhere
The international convention counts completed years since birth, so you are zero until your first birthday. Traditional East Asian age reckoning counted a newborn as one and added a year at each lunar new year, meaning a baby born in December could be counted as two within weeks.
South Korea used a similar system until 2023, when it standardised on the international method for legal and administrative purposes.
Frequently asked questions
Why does my age in days differ from other calculators? +
Usually leap-day handling or timezone drift. This calculator works in whole calendar days from midnight to midnight, so daylight saving transitions cannot shift the result.
Why does my age in days not equal my years times 365? +
Leap days. Over 40 years there are about ten extra days, which is why the exact count runs ahead of the simple multiplication.
How is age calculated for insurance? +
Many insurers use "age nearest birthday" rather than age last birthday, which can make you a year older in their calculation within six months of your birthday.