Exact duration, not an approximation
Working out someone's exact age, or the precise number of days until a deadline, sounds simple until leap years and uneven month lengths get involved. This tool computes it exactly, directly in your browser.
Using it
Enter a start date (a birth date, for example) and an end date, or leave the end date as today. The duration appears as years, months and days, along with total months, weeks and days, and how many days remain until the next birthday or anniversary.
Advantages and limitations
Advantages
- Genuinely exact, accounting for leap years and real month lengths
- Works for any two dates, not just birth dates
Limitations
- Works with calendar dates only, not times of day or time zones
- Assumes the standard Gregorian calendar
Common uses
- Finding someone's exact current age and when their next birthday falls.
- Counting down the exact days until a deadline or event.
- Working out the duration between two historical dates.
Why calendar math isn't simple subtraction
A rough shortcut like "divide the number of days by 365" quietly breaks down around leap years and the genuinely uneven lengths of different months — February's 28 or 29 days versus a 31-day month can shift a naive calculation by a day in certain date combinations. This tool works from actual calendar dates directly, correctly accounting for real month lengths and leap years rather than relying on an averaged approximation.
Pairs well with
For adding up hours worked instead of calendar duration, the site's Timesheet Calculator handles that directly.
Frequently asked questions
+Is this calculator really free, with no sign up?
Yes. There's no account and no limit on how many times you can use it.
+Does it account for leap years?
Yes — the calculation is based on actual calendar dates, so leap years and varying month lengths (28, 29, 30, or 31 days) are all handled correctly rather than assuming every month or year is the same length.
+Why is "how many years old" not just a simple subtraction?
Because months have different lengths and years occasionally have an extra day, naively subtracting birth year from the current year (or even just counting days and dividing by 365) can be off by a day in certain date combinations. This tool works from actual calendar dates directly, avoiding that class of rounding error entirely.
+Is my data sent anywhere?
No. Everything is computed directly in your browser.