Date Calculator
Add or subtract days, weeks, months or years from a date. Find the resulting weekday, ISO week and day of the year, with an optional workday mode.
The example below uses today’s UTC date.
Resulting date
October 21, 2026 → November 20, 2026
| Measure | Value |
|---|---|
| Day of the week | Friday |
| ISO week and week-year | 2026-W47 |
| Day of the year | 324 / 365 |
How the calculation works
Days and weeks are added using UTC calendar arithmetic. For months and years, the original day is kept when it exists; otherwise the last day of the target month is used. January 31 + 1 month = February 28 in a non-leap year, while January 31 + 2 months = March 31. The starting day is excluded when adding workdays.
Frequently asked questions
What happens when I add a month to January 31?
The result is February 28 in a common year or February 29 in a leap year. The calculator clamps to the end of the target month instead of spilling into March.
Does workday mode skip public holidays?
No. It skips Saturdays and Sundays only. One workweek is five working days. Use the business days calculator if Saturday should count as a working day.
Why can the ISO week-year differ from the calendar year?
An ISO week starts on Monday, and week 1 contains January 4. Dates around New Year can therefore belong to the previous or following ISO week-year.
Is adding one year the same as adding 365 days?
Not always. A year can have 365 or 366 days. Adding one year keeps the month and day where possible; February 29 becomes February 28 if the target year is not a leap year.