How the date calculation works
Difference mode compares two calendar dates. Add/subtract mode changes the start date by the selected number of days, weeks, months or years using calendar arithmetic.
Example
From January 1 to January 31, the difference is 30 elapsed days if the start date is not counted. Adding 30 days to January 1 lands on January 31.
What to check before using the result
This calculator uses calendar days and local date input values. Business days, weekends and public holidays are country-specific and are not included in this calculation. Month and year arithmetic can land on adjusted month-end dates when the target month has fewer days.
References
- NIST: UTC(NIST) Time Scale, accessed 2026-05-13.
Last reviewed: 2026-05-14