Method / assumptions / examples
How this calculation works
The result is deterministic: the same measurements always return the same estimate. Here is the relationship and where real-world results can differ.
Formula
Elapsed days = end date − start date; calendar interval = complete years + months + days
Elapsed days compares normalized calendar dates. The calendar interval advances through complete years and months before counting remaining days, so it follows calendar boundaries rather than treating every month as a fixed number of days. Age uses the same completed-anniversary method.
Worked example
Examples
January 15, 2024 to March 20, 2025 spans 1 year, 2 months, and 5 days, or 430 elapsed days.
Someone born July 18, 1990 is 36 years old on July 18, 2026 because 36 complete anniversaries have passed.
Common mistakes
What to check before using the result
- Calendar years and months are variable lengths. Use total elapsed days when a fixed-duration answer matters.
- Check whether your use case counts both the start and end dates. The ordinary difference between two dates excludes the starting instant.
- Legal, benefits, and contract rules can define age or service periods differently; verify the governing convention for consequential decisions.
FAQ
Frequently asked questions
Why is one day not added for both endpoints?
A date difference measures the duration from the start of one date to the start of the other. Inclusive day counts add one when both calendar dates must be counted.
How are leap-day birthdays handled?
The calculator follows calendar anniversaries. Rules for legal age on February 28 versus March 1 can vary by jurisdiction.
Why do months not convert to one fixed number of days?
Calendar months range from 28 to 31 days. The tool reports a calendar interval and a separate total-day duration so the distinction stays visible.