What is the Age in Days Calculator?
This tool tells you exactly how many days have passed between a birth date and any chosen date. Counting age in days is popular for marking thousand-day milestones, tracking a baby's age, planning a special "day number" celebration, or simply satisfying curiosity about how many days you have been alive. It works for any two valid calendar dates and automatically accounts for leap years.
How to use it
Enter the birth date using the Birth Year, Month, and Day fields. Then set the As-of date — the day you want to count up to. By default this is today's date, but you can change it to a past or future date. Press calculate to see the total in days, plus handy conversions into weeks, approximate months, and hours.
The formula explained
The calculator converts both dates to a timestamp (milliseconds since a fixed reference) and subtracts them. Dividing the difference by the number of milliseconds in one day (86,400,000) and taking the floor gives a whole number of completed days:
$$\text{days} = \left\lfloor \frac{\text{as-of date} - \text{birth date}}{1\ \text{day}} \right\rfloor$$
Weeks are \(\text{days} \div 7\), hours are \(\text{days} \times 24\), and months use the average month length of 30.4375 days (\(365.25 \div 12\)) for a leap-year-aware approximation.
Worked example
Suppose someone was born on 1 January 2000 and you want their age as of 1 January 2001. The year 2000 is a leap year (366 days), so the result is exactly 366 days, which equals about 52.29 weeks and 8,784 hours.
FAQ
Does it count leap years? Yes. Because it works from real calendar timestamps, leap days (like 29 February) are included automatically.
Can I use a future date? Yes — set the As-of date to a future date to count how many days remain until a milestone, though the label still reads "days old".
Why is the month figure approximate? Calendar months vary from 28 to 31 days, so months are estimated using the average length of 30.4375 days. The day count itself is exact.
