How to use this calculator
The age calculator works out how old someone or something is on a given date. Enter a birth date and an “age on” date, which starts as today. The result shows the age in years, months and days, then the same span as total months, total weeks and total days. It also shows the weekday of the birth date, the date of the next birthday, the weekday it falls on and how many days remain until it.
Set the “age on” date to any day to find the age at a past or future event, such as the start of a school year, a retirement date or a deadline that depends on age. For someone born on February 29, choose whether the birthday is observed on February 28 or March 1 in years without a leap day. February 28 is the default. The calculator accepts dates from 1900 to 2100.
Formula and calculation method
Age = whole years from the birth date, then whole months, then remaining days, counted on the calendar; totals = the same span in months, weeks or days.
Calendar arithmetic, not average years
The calculator counts complete calendar years from the birth date, then complete calendar months from the last birthday, then the days left over. It uses the actual lengths of months and the actual leap years in between. It does not divide a day count by 365 or 365.25, which would give an age that drifts by a day or more around birthdays. The totals are calculated in the same way: total months counts complete calendar months, total days counts every date step, and total weeks is the day count divided by seven, with any leftover days shown.
Finding the next birthday
The next birthday is the first anniversary of the birth date that falls after the “age on” date. If the “age on” date is itself a birthday, that day is the birthday and the next one is a year later. The calculator gives its weekday and the number of days from the “age on” date to it. For a February 29 birth date in a year without a leap day, the selected option decides whether the birthday falls on February 28 or March 1.
Worked examples
Born May 15, 1990
Someone born on Tuesday, May 15, 1990 is 36 years, 5 months and 14 days old on October 29, 2026. Their 36th birthday was May 15, 2026; five months later is October 15, and 14 more days reach October 29. The same span is 437 complete months, 13,316 days, or 1,902 weeks and 2 days. The next birthday, their 37th, is Saturday, May 15, 2027, 198 days later.
A leap-day birthday
Someone born on Tuesday, February 29, 2000 is 26 years, 8 months and 0 days old on October 29, 2026. Their 27th birthday falls in 2027, which has no February 29. With the default option it is Sunday, February 28, 2027, 122 days away; with the March 1 option it is Monday, March 1, 2027, 123 days away. In 2028 the birthday falls on February 29 again.
A birth date at the end of a month
For a birth date of August 31, 1990, the age on October 29, 2026 is 36 years, 1 month and 29 days. One month after August 31, 2026 would be September 31, which does not exist, so the month is completed on September 30, the last day of September. From September 30 to October 29 is 29 days. The total span is 13,208 days, or 1,886 weeks and 6 days.
Edge cases and practical checks
Legal and official ages
The result is calendar arithmetic. Laws and institutions sometimes use their own rules, such as treating a person born on February 29 as reaching an age on March 1, counting age from the day before a birthday, or using a cut-off date such as September 1 for school entry. Some cultures count age differently, for example starting at one at birth. Check the rule that applies before relying on the result for an official purpose.
Input range and order
Both dates must be valid Gregorian dates between 1900 and 2100, and the “age on” date should be on or after the birth date for the result to describe an age. The calculator works with dates only, not times of birth or time zones, so it cannot tell whether someone born late in the evening in one country has reached a birthday at a given hour in another.
Frequently asked questions
How is age calculated in years, months and days?
The calculator finds the most recent birthday on or before the “age on” date, which gives the years. It then counts complete months from that birthday, and finally the remaining days. Because months have different lengths, the days part depends on which months are involved, not on a fixed 30-day month.
Why does total days not equal years times 365?
Leap years add a day each, and the months and days part of the age covers a varying number of days. Total days counts each calendar date between the two dates. For a 36-year span, about nine leap days are usually included.
When does someone born on February 29 have a birthday?
In leap years, on February 29. In other years the calculator uses February 28 by default, or March 1 if you choose that option. The choice changes the next birthday shown and the days until it. Choose the convention you follow, or the one an organisation requires.
Can I find my age on a past or future date?
Yes. Change the “age on” date to any date from the birth date up to 2100. This is useful for checking an age requirement on a specific day, such as an event date, rather than today.
How do I find the day of the week I was born?
Enter your birth date. The result includes its weekday. The calculator uses the Gregorian calendar for all dates from 1900, so the weekday matches modern calendars.
Can I use it for something other than a person?
Yes. Any two dates work: the age of a building, a company, a pet or an account. The result is the calendar span between the two dates, and the next “birthday” is the next anniversary of the first date.
Does the time of birth matter?
Not for this calculator. It compares dates without times, and a birthday counts from the start of that date. If you need the exact elapsed time between two moments, including hours, use a duration calculator with dated times.