Age / Date Difference
How the age calculation works
Enter a date of birth and an “as of” date to calculate calendar years, months and days, total elapsed days, and the next birthday. The calculation runs locally in your browser and does not transmit the dates you enter.
Formula and method
Calendar age borrows days from the preceding month and months from the preceding year, so it follows calendar boundaries rather than dividing elapsed days by 365.
Worked example
Someone born on January 1, 2000 is 26 years old on January 1, 2026, regardless of the leap days within that interval.
How to interpret the result
Calendar age is not the same as elapsed time divided by 365. The year, month, and day result follows actual calendar boundaries, including months of different lengths and leap years. Total days is useful for elapsed-time comparisons, while calendar age is usually the form expected for birthdays, eligibility dates, and anniversaries.
Important limitations
- Results use the dates and time zone reported by your device.
- Legal age rules can vary by jurisdiction.
Common questions
How are February 29 birthdays handled?
The calculator follows calendar arithmetic around the selected as-of date. Rules for legal birthdays in non-leap years can differ by jurisdiction, so use the displayed result as a date calculation rather than a legal determination.
Why might the answer differ by one day?
Dates are interpreted using the browser's local calendar and time-zone behavior. Confirm both entered dates and avoid converting a date through a timestamp in another time zone before entering it.
Related tools
Method and examples reviewed September 8, 2026.