Birthday Calculator

What the birthday calculator does

This takes your birthday and works out exactly how old you are. You enter the date you were born and the date to count up to, and it gives you your age broken down into years, months, weeks, and total days. The birthday is the anchor, and everything follows from it, so it is the quick way to turn a date of birth into a precise age rather than a rough one.

How to use it

1. Enter the Birthdate. The day you were born.
2. Calculate Age As Of. The date to measure the age up to, which can be today or any other day.

Press Calculate and your age appears, broken into years, months, weeks, and a total in days.

The full breakdown, not just a number

Most of the time age gets boiled down to a single number of years, which is fine for a form but loses a lot. This shows the whole picture: the years, then the extra months and weeks on top, and the grand total in days. That extra detail is what you want when the exact figure matters, like working out a precise age for a young child where months and weeks still count for a lot, or simply seeing how many days you have actually been around, which is a surprisingly large and satisfying number.

Your age on any date you choose

The second date, the one you are counting up to, is more useful than it first looks, because it does not have to be today. Set it in the future and you can see how old you will be on a particular day, like a wedding, an exam, or your next birthday. Set it in the past and you can find how old you were when something happened. So the same tool answers how old am I, how old will I be, and how old was I, just by changing that second date. It turns a simple age check into something you can point at any moment in your life.

If your birthday is on the 29th of February

Being born on the 29th of February makes for an unusual birthday, since that date only exists in leap years, which come around every four years. People born then are sometimes called leaplings, and in common years their birthday is usually marked on either the 28th of February or the 1st of March. Your age still climbs by one each year like everyone else's, counted from the day you were born, so the calculator gives your true age in years and days even in the three years out of four when your actual birth date does not appear on the calendar. It just means your real birth date shows up to be celebrated only once every four years.

How it works

Working out an age sounds like simple subtraction, but it has to respect the calendar, where months are different lengths and leap years add a day, so a careful tool does not just divide the days by 365. This one uses Moment.js, a well-established date and time library, to handle that calendar arithmetic, which is how it gets the years, months, and the leap-year cases right rather than approximating them.

Questions people ask

How do I work out my exact age from my birthday?

Enter your date of birth and the date to count to, and the tool returns your age in years, months, and weeks, plus the total number of days.

Can it tell me how old I will be on a future date?

Yes. Set the second date in the future, such as your next birthday, and it shows the age you will be on that day.

How many days old am I?

The result includes your age as a total in days, which is usually a much larger and more interesting number than the figure in years.

What if I was born on the 29th of February?

Your age is still counted from your birth date, so the result is accurate every year, even though your actual birth date only lands on the calendar in leap years.

References

1. Moment.js, JavaScript date and time library. https://momentjs.com/
2. U.S. Naval Observatory, Astronomical Applications Department: Leap Years. https://aa.usno.navy.mil/faq/leap_years