How age is calculated

The counting rules behind every calculator on 123AgeCalculator, with examples you can check.

Age is counted on the calendar: first the whole years up to your last birthday, then the whole months since it, then the days since the last monthly anniversary. Born June 15, 1990, on October 30, 2026 you are 36 years (to June 15, 2026), 4 months (to October 15) and 15 days old. When a month has no such day — 31 April, 30 February — the anniversary is the 1st of the next month, which is also how a 29 February birthday is counted in other years.

The rule

  1. Years: count the birthdays you have had. Born June 15, 1990, your 36th birthday was June 15, 2026.
  2. Months: from that birthday, count the times the same day of the month has come round — July 15, August 15, September 15, October 15 — 4 months.
  3. Days: count the days from the last of those, October 15, to the date you are measuring to, October 30 — 15 days.

The answer, 36 years, 4 months and 15 days, is what a person, a passport office or a doctor means by an age. The same rule runs every calculator on the site, so the age on one page always agrees with another.

Why not divide by 365?

Because years and months are not all the same length, dividing a number of days gives the wrong answer near a birthday:

  • Someone born on March 1, 1999 has lived 1,460 days on February 28, 2003. Divide by 365 and they are 4 — a day early, because the leap day of 2000 is in between. By the calendar they are 3 years, 11 months and 27 days.
  • Divide by 365.25 instead, and a child born on January 1, 2001 is still only 0.9993 years old on their first birthday — 365 days later.

Dividing is fine for a rough figure, and the calculators use it only where a decimal is the point — "≈ 19.6 months", "19.34 Mars years" — never for the age in years, months and days.

Month ends

Months have 28 to 31 days, so a birth date late in the month does not exist in every month. The rule: when the anniversary day does not exist in a month, it falls on the 1st of the following month.

FromToAgeWhy
January 31February 2828 daysFebruary has no 31st, so the first month is not yet complete
January 31March 11 monththe missing February 31 counts as March 1
January 31March 312 monthsback to the 31st
February 28March 311 month, 3 daysa month to March 28, then 3 days
March 31April 3030 daysApril has no 31st; the month is complete on May 1

Other conventions exist. Some software instead moves the anniversary back to the last day of the short month, and some moves it forward by the missing days. The differences only ever affect births on the 29th, 30th or 31st, by a day or two, and the rule here has the advantage of being the same one used for leap-day birthdays below.

Born on 29 February

A leap year has 366 days: every year divisible by 4, except century years not divisible by 400. 2000 was a leap year; 1900 was not, and 2100 will not be.

Someone born on February 29 has a real birthday only every four years. In other years, this site treats their birthday as March 1 — the first day on which a full year has passed — so on February 28, 2025 a baby born on February 29, 2024 is 11 months and 30 days old, and turns 1 on March 1. Laws differ: some countries count a leap-day birthday as February 28 in common years for legal purposes. If the exact day matters legally, check the rule where you live.

Days, weeks, hours and seconds

  • Days are counted between the two dates — the day of birth is day 0, so a baby is 1 day old the day after it is born. "Day of life" counts the day of birth as day 1, so it is always one more.
  • Weeks are the days divided by 7, with the remainder in days.
  • Hours, minutes and seconds are counted from midnight on the day of birth unless you give a time of birth. The live counter on the age in seconds calculator uses your device's clock.
  • Total months are the years × 12 plus the months, with the remaining days; a decimal figure divides those days by the length of the month in progress.

What "today" means

"Today" is your date, read from your own device, so someone in Tokyo and someone in New York each get their own answer at midnight their time. Every result can also be calculated to any other date — past or future — with the "Age on" box. Ages are counted in whole calendar days; the time zone of the place of birth only matters for the seconds counter, which assumes the time of birth was in your current time zone.

Other ways of counting age

  • Korean age. The traditional count starts at 1 at birth and adds a year every January 1. South Korea made international age — the method above — the legal standard on June 28, 2023, keeping "year age" (current year minus birth year) for a few laws. See the Korean age calculator.
  • Chinese zodiac years run from Lunar New Year, not January 1, so the zodiac calculator uses the date of Lunar New Year for every year from 1900 to 2100.
  • Chronological age for tests is found by subtracting the dates column by column, borrowing a month (as 30 days, or the real month length, depending on the test) when needed. The chronological age calculator shows the working and lets you choose.
  • Corrected age for a premature baby subtracts the weeks born early, and is used for development checks until about age 2. See the baby age calculator.
  • Social Security treats you as reaching an age on the day before your birthday, which is why someone born on the 1st of a month is counted as born in the previous month. See the retirement age calculator.

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