What Is the Days Old Calculator?
This tool tells you exactly how many days have passed between two calendar dates — most commonly your date of birth and today. It also converts that span into total weeks and total hours, giving you a fun and precise way to measure age or any time interval.
How to Use It
Enter your birth date (or any start date) and an end date. The end date defaults to today. Submit the form and the calculator returns the number of whole days between the two dates, plus the equivalent in weeks and hours.
The Formula Explained
The calculator converts both dates to a millisecond timestamp at midnight, subtracts the birth date from the end date, then divides by the number of milliseconds in a day (1000 × 60 × 60 × 24 = 86,400,000) and rounds down:
$$\text{Days Old} = \left\lfloor \frac{\text{End Date} - \text{Birth Date}}{1\ \text{day}} \right\rfloor$$Weeks = floor(Days ÷ 7), and Hours = Days × 24.
Worked Example
Suppose you were born on 2000-01-01 and the end date is 2020-01-01. That span contains 20 years, including 5 leap days (2000, 2004, 2008, 2012, 2016). So the total is \(20 \times 365 + 5 = \mathbf{7{,}305}\) days. That equals \(\lfloor 7305 \div 7 \rfloor = 1{,}043\) weeks and \(7{,}305 \times 24 = 175{,}320\) hours.
FAQ
Does it count the end date? The result is the number of full days elapsed, so the start date counts as day 0 and the end date is reached after the listed days.
Are leap years included? Yes. Because we use real calendar timestamps, every leap day in the range is automatically counted.
Can I use it for any two dates? Absolutely — it works for any start and end date, not just birthdays.
