Distinguish elapsed days from included dates
Days between dates subtracts the start date from the end date. January 1 to January 2 is one elapsed day, while counting both calendar dates gives two. Establish which question you need to answer before using a total. An interval is not automatically the number of dates included in a schedule.
Add days shifts a calendar date by an integer: ten days after January 1 is January 11, and negative values subtract days. These tools calculate calendar dates using UTC to avoid differences caused by clock changes. They do not calculate working hours or interpret the special rules of a particular scheduling calendar.
Count working days with explicit exclusions
Working days treats Saturday and Sunday as weekends. It includes the start date and lets you include or exclude the end date. Enter holidays in YYYY-MM-DD format, one per line. No official calendars are downloaded, so you must supply the exclusions relevant to your task.
Repeated holidays are deducted once, and holidays falling on weekends are not deducted again. Test a known week with a middle weekday excluded to check the behavior. If your activity works Saturdays or follows another weekend pattern, this model does not represent that calendar. Verify the convention before applying the total to a real plan or schedule.
Interpret ISO weeks and timestamps
ISO week reports year, week and weekday. Weeks begin on Monday and week one contains the first Thursday of the year. Near January and December, the ISO year can differ from the calendar year. January 1, 2021 is 2020-W53-5: Friday of ISO year 2020’s week 53.
A Unix timestamp represents an instant rather than only a calendar date. Select seconds or milliseconds, and include Z or an offset such as +02:00 when entering a date and time. Output is in UTC. Different local clock readings can represent the same instant when their offsets differ, so preserve the offset in your source records.
Test boundaries and document the convention
Try a single-date range, a weekend and an interval with the end excluded. Record whether the answer measures elapsed days or included dates. For fictional dates, the random generator includes both endpoints and can avoid repetitions when there are enough dates available within the selected range.
Random dates are not filtered for working days or holidays. A test list can contain Saturdays, Sundays and closures your activity excludes. Retain the convention alongside the result and check format and units before copying into another application. For a specific calendar, independently validate its rules and the exclusion dates you supplied rather than treating a general date calculation as that calendar’s authority.
A week with one closure
Enter these dates in Working days. The exclusion is fictional for this trial and does not represent an official holiday calendar. Calculate with the end included, then exclude it to observe the counting convention.
Start: 2026-10-12 End: 2026-10-16 Sample exclusion: 2026-10-14 Include end date: yes
Including the end gives five dates and four working days after excluding Wednesday. Excluding Friday gives four dates and three working days.
Days between dates gives four elapsed days for this interval. ISO week reports 2020-W53-5 for 2021-01-01. Compare meaning as well as numbers before choosing which result answers your question.