Date arithmetic is tested on every build against hand-worked cases, including month ends, leap days and two-digit years. How it is checked
Enter a date of birth to get an age in years, then whole months, then days, the way a person says it, along with the total days lived and the next birthday. A 29 February birthday is the one contested case, and for it you choose which rule applies.
Why an age has more than one right answer
For most people it has one: you are a year older on your birthday, and between birthdays you count whole months and then the days left over. The exception is people born on 29 February, who have a real birthday only once every four years, and jurisdictions differ on what happens in the other three.
Much of the United States treats the birthday as falling on 28 February in a common year, which never delays a coming-of-age. England and Wales, among others, take 1 March, on the reasoning that a year is not complete until the corresponding month has run out. Both rules are in force somewhere, and they are a full day of legal age apart on the date it matters most. This page offers both and names the one it used.
Why months are counted one at a time
The quick way to compute an age in months is to divide the day count by 30.44 and round. That goes wrong because months differ in length, so the error shifts as you move through the calendar. This page counts whole months by stepping through them one at a time from the last birthday, then reports the remaining days exactly.
The day before your birthday
On the day before a birthday you are still the younger age. A calculator that gets this wrong looks correct for three hundred and sixty-four days a year, so the site's tests check this case on every build: someone born on the 25th is one age on the 24th and the next age on the 25th.
Questions
How old am I exactly?
Enter your date of birth and the date you want the age on (today, by default), and the answer above gives years, whole months and remaining days, along with the total number of days you have been alive. All three parts are exact. The months are counted one at a time; estimating them from an average month length is the usual source of an answer that is a day out.
What happens if I was born on 29 February?
You choose, because jurisdictions differ: much of the United States uses 28 February in a common year, so you reach each legal age at the earliest defensible moment, and England and Wales use 1 March. In a leap year both rules agree it is the 29th. A control for this appears above when the date of birth is 29 February, and the answer names the rule it used.
Does it count the day I was born?
No. Your age is zero on the day you are born and one on your first birthday, which is the ordinary human reading and the one used here. The separate “total days” figure is a plain count of days elapsed since birth, so it is also zero on the day you are born. Some medical and legal contexts count differently; where that matters, the total-days figure is usually the number you want.
Why does my age in months not match my age in years times twelve?
Because the months figure counts whole months since your last birthday, so it is always between zero and eleven. Thirty-six years and five months means thirty-six full years, and then five more whole months on top. For the total number of months you have been alive, multiply the years by twelve and add the months.
Is anything I type here sent anywhere?
No, and not as a matter of policy. This page is a static file and a script your browser downloads once; there is no server behind it to receive anything. Your date of birth is read by the script in your own browser and the answer is written back into the same page. Nothing is sent, nothing is stored, and no analytics or third-party script runs here.
Dates and Days