Age Calculator

DateMath counts the whole years since the date of birth, then the whole months since the last birthday, then the days left over. Months are calendar months, not 30-day blocks, and a February 29 birthday falls on February 28 in years without a February 29.

• The Gregorian calendar. DateMath treats every date as a Gregorian date, with its leap-year rule: a year divisible by 4 is a leap year, except century years not divisible by 400, so 2000 had a February 29 and 2100 will not. Source: Gregorian calendar.

DateMath's date math is tested against more than 100 cases worked out by hand. Every worked example and table on the site is computed with the same code the calculators run, so a page and its calculator can't disagree.

Your age is the whole years since your date of birth, then the whole months since your last birthday, then the days left over. The calculator fills in today’s date from your device, so it answers “how old am I today” as soon as you enter a date of birth; change “Age on this date” for any other day.

The method, step by step

Born May 17, 1990, on January 1, 2027:

1. Subtract the birth year from the year: 2027 − 1990 = 37.
2. If the birthday has not come yet that year, subtract one. The 2027 birthday is still ahead, so the age is 36 years, counted from the last birthday on May 17, 2026.
3. Count the whole months from the last birthday: up to December 17, 2026 is 7 months.
4. Count the days left: from December 17, 2026 to January 1, 2027 is 15 days.

The age is 36 years, 7 months, 15 days. The same date of birth is 439 months, 1,911 weeks, 1 day, or 13,378 days old on that date. The calculator does all of this at once and runs in your browser.

With only the birth year there are two answers for any date: one before that year’s birthday and one from the birthday on. Subtract the birth year from the current year for the age after the birthday, and one more for the age before it. Born in 1999: 27 years old until the 2027 birthday, 28 from it. Enter the full date of birth to know which one applies today.

Below the age, the calculator shows the same span in single units. Total months counts every whole month since the birth date, so it matches the years and months of the age. Total weeks gives whole weeks plus the days left over. Total days counts every day from the date of birth to the chosen date, February 29s included; the day of birth itself is day zero.

To find the date you reach a round number, add it to your date of birth in the date calculator, in days, weeks or months.

It depends on how many February 29s the span contains: a year of age is 365 days, or 366 when it includes a leap day. From January 1, 2027, 15 years is 5,479 days, 4 leap days included. The table counts other spans from the same date; for your own dates, use the date calculator.

2100 is not a leap year: century years are leap years only when they divide evenly by 400, so the longest span has one leap day fewer than “every fourth year” suggests.

The last two result rows answer the usual birthday questions. “Born on a” is the weekday of the date of birth. “Next birthday” is the date of the next one after the chosen date, the age you will turn and how many days are left; when the chosen date is the birthday itself, the row says so.

Leap years

Ages are counted on the calendar, not as a fixed number of days per year, so every February 29 lived is in the totals. Born May 17, 1990, the 13,378 days to January 1, 2027 include 9 February 29s.

February 29 birthdays

In years without a February 29, DateMath counts the birthday on February 28. Laws differ on this point (some count March 1), so check the local rule when the exact day matters for a legal purpose. Born February 29, 2000:

• On February 28, 2027: 27 years, a birthday.
• On March 1, 2027: 27 years, 1 day.
• On February 29, 2028: 28 years, back on the real date.

Birthdays at the end of a month

When a month has no day matching the birth date, that month is complete on its last day and the days are counted from there. Adding the result back to the date of birth always lands on the chosen date. Born January 31, 2000:

• On February 28, 2027: 27 years, 1 month.
• On March 30, 2027: 27 years, 1 month, 30 days.
• On March 31, 2027: 27 years, 2 months.

Enter the older person’s birth date as “Date of birth” and the younger person’s as “Age on this date”: the result is the age gap in years, months and days, with the totals below it. Born September 3, 1988 and April 22, 1991, the gap is 2 years, 7 months, 19 days (961 days).

Their ages in whole years differ by one more from the older person’s birthday until the younger one’s. On January 1, 2027 they are 38 and 35; on June 1, 2027, 38 and 36. For the gap down to the minute, enter both birth times in the duration calculator.

• Cutoff dates. School entry, youth sports and contests often go by the age on a set date; enter that date as “Age on this date”. The organization’s own rule decides, including for February 29 birthdays.
• Forms and records. Age at the time of a past event, such as a move, a first job or a family photo.
• Babies and toddlers. Young children’s ages are usually given in weeks or months; the totals give both.
• Milestones. The next birthday, a round number of days, or the date you turn a given age: add the years to the date of birth in the date calculator.