Date arithmetic is different from adding a fixed number of 24-hour periods. Month lengths and leap years affect the result.
Enter the starting date
Choose the calendar date from which the calculation begins. Then select whether to add or subtract years, months and days.
Month-end behavior
If the target month does not have the original day, calendar calculations clamp to the last valid day. This keeps results practical for dates such as January 31.
Use the result in context
Contracts, billing cycles and eligibility rules may define dates differently. Check whether a deadline includes or excludes the starting date.
- Confirm the year and leap-year status.
- Read the operation mode carefully.
- Check the official rule for deadlines.
Frequently asked questions
What happens when I add one month to January 31?
The result is clamped to the last valid day of February for that year.
Is date arithmetic the same as counting days?
No. Adding one month is calendar arithmetic, while counting days uses the exact difference between dates.