Dates and time
Age Calculator
Work out exact age in years, months, and days from a date of birth, on today's date or any other, with the next birthday, age in months, weeks, and days, and the age gap between two people.
Maintained by Austin Witherow · Updated
Formula
Whole months = (Age-on year − Birth year) × 12 + (Age-on month − Birth month), minus 1 if that monthly date is after the age-on dateYears = Whole months ÷ 12, rounded down; Months = the remainderDays = Age-on date − the last monthly date of birthTotal days = Age-on date − Date of birthWeeks = Total days ÷ 7; Hours = Total days × 24; Minutes = Total days × 1,440; Seconds = Total days × 86,400
Key caveat
Ages count whole days from midnight; the time of birth is not used.
What this calculator is for
Age in years, months, and days is the number of whole years since the last birthday, the whole months since the last monthly date of birth, and the days since that. This calculator works it out on today's date or any other, gives the next birthday and how far away it is, and turns the same span into total months, weeks, days, and hours. Switch to Age gap to compare two dates of birth.
Months have 28 to 31 days, so it counts real calendar months; a birth on the 31st has its monthly date on the last day of shorter months. Someone born on February 29 has their birthday counted on February 28 in other years; the result also shows the age if it is counted on March 1, since conventions differ.
How it works
Years and months: count the whole months from the date of birth to the age-on date, stepping back one if the monthly date has not come yet. Divide by 12 for years; the remainder is months.
A birth on the 29th, 30th, or 31st has its monthly date on the last day of shorter months: born January 31, the February date is February 28 (or 29 in a leap year).
Days: the days from the last monthly date to the age-on date.
Totals: the age-on date minus the date of birth in days; weeks are days ÷ 7; hours are days × 24, minutes days × 1,440, and seconds days × 86,400, counting from midnight.
The leap-year rule follows the Gregorian calendar as the U.S. Naval Observatory states it: every fourth year, except century years not divisible by 400.
The fixed 30-day option instead subtracts year, month, and day separately, borrowing 30 days for a short day count and 12 months for a short month count.
How the age is counted
Top: born June 15, 1990, measured on October 8, 2026: whole years to the last birthday (June 15, 2026), whole months to the last monthly date of birth (September 15), then the days left (23). Result: 36 years, 3 months, 23 days. Bottom: born January 31, 2000, measured on March 1, 2026: February has no 31st, so the monthly date is February 28, and the age is 26 years, 1 month, 1 day. Not to scale.
Quick calculator insights
Months are not all the same length
- Born on the 31st
- The monthly date falls on the last day of 30-day months and February.
- Born February 29
- Counted on February 28 in other years here; the result also shows the age counted on March 1.
- Fixed 30-day months
- Can differ from calendar months by a day or two near month ends; use them only if a test manual specifies them.
Leap years around today
U.S. Naval Observatory rule: divisible by 4, except century years not divisible by 400.
| Year | Leap year? | Why |
|---|---|---|
| 1900 | No | A century year not divisible by 400 |
| 2000 | Yes | Divisible by 400 |
| 2024 | Yes | Divisible by 4 |
| 2026 | No | Not divisible by 4 |
| 2028 | Yes | Divisible by 4 |
| 2100 | No | A century year not divisible by 400 |
Ways to count age
The same person can have different ages depending on the rule.
| Count | Starts at | Goes up | Used for |
|---|---|---|---|
| International age | 0 at birth | On each birthday | Most of the world; South Korea's official count since June 28, 2023 |
| Traditional Korean age | 1 at birth | Every January 1 | Everyday use in Korea; no longer official |
| Calendar age (by year of birth) | 0 in the birth year | Every January 1 | Some Korean rules, such as alcohol and tobacco |
| Fixed 30-day months | 0 at birth | As calendar-month age, but months borrow 30 days | Only where a test manual specifies it |
How to use this calculator
- 01Enter the date of birth.
- 02Leave Age on as today, or set the date you need an age for.
- 03Read the age, the next birthday, and the totals.
- 04For two people, choose Age gap and enter both dates of birth.
Inputs explained
- Calculate
- Age on a date, or the age gap between two dates of birth.
- Date of birth
- The day someone was born, or the start of any span.
- Age on
- The date to measure the age on. It starts as today, on your device's calendar.
- Second date of birth
- For Age gap: the other person's date of birth.
- Count months by
- Calendar months by default. The fixed 30-day option is for test manuals that specify it.
Outputs explained
- Age
- Whole years, months, and days on the age-on date; for Age gap, the difference between the two dates of birth.
- Next birthday
- The weekday and date of the next birthday after the age-on date, the age turned, and how many days after the age-on date it falls.
- Age in months, weeks, days, hours, minutes, seconds
- The same span as single totals. Hours, minutes, and seconds count from midnight on both dates, since the time of birth is not entered.
- Next half birthday
- Six months after a birthday, on the same day of the month (the last day if the month is shorter).
- Day of the week born
- The weekday of the date of birth.
- Leap-day birthday
- For a February 29 birth: the age with the birthday counted on February 28 in other years (used above), and with it counted on March 1.
- Korean counts
- Traditional Korean age (1 at birth, plus a year every January 1) and calendar age by year of birth, which some Korean rules still use.
- Who is older, and the gap
- For Age gap: which date of birth is earlier, by how much, and the gap in months, weeks, and days.
Worked examples
Born June 15, 1990, on October 8, 2026
- Whole months: 36 × 12 + 4 = 436, but October 15 has not come yet, so 435: 36 years and 3 months, to September 15, 2026.
- Days from September 15 to October 8: 23. Age: 36 years, 3 months, 23 days.
- Total: 13,264 days, or 1,894 weeks and 6 days: about 19,100,160 minutes or 1,146,009,600 seconds. Born on a Friday.
- Next birthday: Tuesday, June 15, 2027, turning 37, in 250 days. Next half birthday: December 15, 2026, in 68 days.
Born January 31, 2000, on March 1, 2026
- The monthly date in February 2026 is February 28, the last day of the month. From there to March 1 is 1 day.
- Age by calendar months: 26 years, 1 month, 1 day.
- With fixed 30-day months: 1 − 31 = −30, plus 30 = 0 days, so 26 years, 1 month, 0 days. The two methods differ by a day.
Born February 29, 2004
- 2026 has no February 29, so this calculator counts the birthday on February 28: on February 28, 2026, the age is exactly 22 years.
- On February 27, 2026, it is 21 years, 11 months, 29 days. Counted on March 1 instead, the 22nd birthday is Sunday, March 1, 2026, and on February 28 the age is still 21; the result shows both.
Age gap: March 3, 1988 and November 20, 1991
- From March 3, 1988 to November 20, 1991: 3 years, 8 months, 17 days, or 1,357 days.
- The first person is older by that much.
Korean counts: born December 31, 2000, on January 1, 2026
- International age: 25 years and 1 day.
- Traditional Korean age: 2026 − 2000 + 1 = 27. Calendar age by year of birth: 2026 − 2000 = 26.
When to use it
- Finding someone's exact age today or on a past or future date.
- Counting down to a birthday or a half birthday.
- Working out chronological age on a test date.
- Comparing the ages of two people.
Tips
- For an age cutoff, set Age on to the cutoff date and read the years.
- For a test, set Age on to the test date and use the counting method your manual names.
- To check "how old will I be in" a year, set Age on to a date in that year: before the birthday the age is one less.
Limitations
- Ages count whole days from midnight; the time of birth is not used.
- Age on uses your device's calendar for today, so it follows your time zone.
- How a February 29 birthday is counted in other years differs; this calculator shows both conventions but does not decide legal age.
- Dates use the Gregorian calendar for every year, including years before countries adopted it.
Methodology & assumptions
- Dates are calendar dates with no time zone; differences are counted in whole days.
- The leap-year rule follows the U.S. Naval Observatory's statement of the Gregorian calendar.
- Calendar months step from the date of birth to the same day of later months, clamped to the last day of shorter months.
- Korean counts follow the 2023 change reported by The Korea Times, with the exceptions CNN reported.
Sources
Last reviewed 2026-10-08
- U.S. Naval Observatory: Leap Years
Under the Gregorian calendar, years evenly divisible by 4 are leap years, except centurial years not evenly divisible by 400: 1700, 1800, 1900, and 2100 are not leap years; 1600, 2000, and 2400 are. The average year is 365.2425 days.
- The Korea Times (June 26, 2023): Korea's age system to change from Wednesday
The Ministry of Government Legislation announced that from June 28, 2023, revisions to the Basic Administrative Act and the Civil Act adopt international age. Traditional Korean age makes a person 1 at birth and adds a year every Jan. 1.
- CNN (June 27, 2023): South Koreans become younger overnight after country scraps 'Korean age'
Some old counts continue: children enter elementary school in March of the year after they turn 6, and laws on alcohol and tobacco go by the year someone is born (calendar age), regardless of month. Many people still use traditional Korean age in daily life.
- Microsoft Support: DATEDIF function
DATEDIF(start_date, end_date, unit): "Y" complete years, "M" complete months, "D" days, "YM" months ignoring years. Microsoft does not recommend "MD", which may give a negative, zero, or inaccurate result.
FAQs
How do I calculate my age?+
Count the whole years since your last birthday, then the whole months since your last monthly date of birth, then the days since that. Born June 15, 1990, you are 36 years, 3 months, 23 days old on October 8, 2026.
How old will I be in 2030?+
Subtract your birth year from 2030 for your age after your birthday that year; before it, you are one year younger. Born in 1990, you are 39 until your 2030 birthday and 40 after it.
How old is someone born in 2009 in 2025?+
15 before their 2025 birthday and 16 after it: 2025 − 2009 = 16. Born June 1, 2009, they were 16 years, 6 months, 30 days old on December 31, 2025.
What year was I born if I am 21?+
Subtract your age from the current year, then one more if your birthday has not come yet this year. In 2026, a 21-year-old was born in 2005 if they have had their birthday this year, or 2004 if not.
How do I find a date of birth from an age?+
Count back the years, months, and days from the date the age was measured. Someone who is 36 years, 3 months, 23 days old on October 8, 2026 was born June 15, 1990; check it by entering that date of birth here.
How many days old am I?+
The days between your date of birth and today, which this calculator shows as Age in days. Born June 15, 1990, you are 13,264 days old on October 8, 2026.
How many seconds old am I?+
Your age in days times 86,400, counted from midnight, since this calculator does not use the time of birth. Born June 15, 1990, that is about 1,146,009,600 seconds on October 8, 2026, or about 19,100,160 minutes.
When will I be 10,000 days old?+
Add 10,000 days to your date of birth: about 27 years, 4 months, and 16 days. Born June 15, 1990, the 10,000th day was October 31, 2017.
How old am I in months?+
The whole months since birth, shown as Age in months: 435 months for someone born June 15, 1990, on October 8, 2026.
What day of the week was I born?+
Enter your date of birth: the calculator shows the weekday. June 15, 1990 was a Friday.
When is my half birthday?+
Six months after your birthday, on the same day of the month, or the last day of the month if it is shorter. Born June 15, the half birthday is December 15; born August 31, it is the last day of February.
How do I find the age gap between two people?+
Choose Age gap and enter both dates of birth. March 3, 1988 and November 20, 1991 are 3 years, 8 months, 17 days apart, or 1,357 days.
When does someone born on February 29 have a birthday?+
Every four years on February 29. In other years conventions differ: some count it on February 28, the last day of February, and some on March 1. This calculator uses February 28 and also shows the age counted on March 1. For a legal age, check the rule that applies.
Which years are leap years?+
Years divisible by 4, except century years not divisible by 400, per the U.S. Naval Observatory. 2000 and 2024 were leap years; 1900 was not, and 2100 will not be.
What is chronological age?+
Age in years, months, and days on a given date, often a test date. Enter the date of birth and set Age on to the test date. If your test manual specifies borrowing a fixed 30 days when the days come up short, choose that option; otherwise use calendar months.
What is Korean age?+
Traditional Korean age makes a person 1 at birth and adds a year every January 1. South Korea made international age the official count on June 28, 2023, The Korea Times reported, though some rules, such as those for alcohol and tobacco, still go by year of birth (calendar age), and many people still use the traditional count day to day.
How do I calculate age in Excel?+
Use DATEDIF(birth_date, TODAY(), "Y") for whole years, "YM" for the months after them, and "M" for total months. Microsoft does not recommend the "MD" unit for the remaining days, as it can give a negative, zero, or inaccurate result.
Why do age calculators sometimes disagree by a day?+
Near the end of a month they may count months differently. This one uses real calendar months, with a birth on the 31st falling on the last day of shorter months; a fixed 30-day count, or one that starts from the end of the month, can be a day or two off. Because of the clamping, someone born January 28, 29, 30, or 31 is 1 month, 0 days old on February 28 in a non-leap year.
This calculator counts calendar dates. It does not decide legal age, eligibility, or benefits; check the rule that applies.