Understanding age
The years between two days.
Calendar arithmetic, with a couple of traps in plain sight.
The day-of-month rule.
The conventional way to compute age is to count whole years between the two dates, then count the leftover months and days. The boundary that matters is the day of the month: you turn a year older on the same day of the month as you were born. Born on the 14th, you become a year older on the 14th — regardless of which weekday that falls on, regardless of how many days the intervening months had.
age = years between birthday and today, counting whole anniversaries
Leap-day birthdays — 29 February.
Roughly 1 in 1,461 people are born on 29 February. Most legal systems recognise their "birthday" in non-leap years as either 1 March (the strict day-of-month rule fails, so move forward to the next valid date) or 28 February (round back to the previous valid date). The choice depends on the country and even the law — driving licences and lottery entry age sometimes differ. This calculator uses the 1 March convention, which matches most software libraries.
Calendar age vs. elapsed time.
A 30-year-old has lived approximately 30 × 365.2425 = 10,957 days. But a person born on 1 March 2000 turning 30 on 1 March 2030 has lived exactly 10,957 days, while someone born one day later has lived 10,956. Calendar age and total-days don't move in lockstep — they're related but not identical, and the breakdown rows above show both so the difference is visible.
Why total-months is approximate.
Months don't have a fixed length. A "total months" figure is only ever a rough conversion: 30.4375 days per month on average, but actual months range from 28 to 31. The breakdown above counts whole calendar months elapsed — January to February is one month, regardless of whether that's 28 or 31 days — which is the closest answer to what people usually mean.
Eastern age conventions.
Several East Asian traditions count age differently. In Korean age, you are one year old at birth and gain a year on every Lunar New Year — which means a baby born in December is two years old by mid-January. Korea legally retired this system in 2023, but it still appears in casual speech. Similar conventions appear in Vietnam, China and Japan. This calculator uses the international Western convention throughout: zero at birth, one on the first anniversary.
Days until the next birthday.
The "days until next birthday" line above is the gap from today to the next anniversary of the birth date — which lands either later this year or in the year ahead. Leap-year oddities apply: someone born on 29 February reads "1 March" on most years and "29 February" once every four. The number is correct on either calendar.