What this tool does
The date calculator has two jobs. First, it counts the time between any two dates: the exact number of days, how many full weeks that is, how many of those days are business days (Monday to Friday), and how many full calendar months fit in the range. Second, it does date arithmetic: give it a starting date and a number of days to add or subtract, and it tells you the resulting date and which day of the week it falls on.
How the day count works
The count is the number of midnights between the two dates, which is the convention most date calculators use. From Monday to the next Monday is 7 days, and from January 1 to January 2 is 1 day. Each date is converted to a UTC timestamp and the difference is divided by 86,400,000 (the number of milliseconds in 24 hours), which keeps daylight saving transitions from ever shifting the answer.
Worked example
From 2026-03-01 to 2026-03-15 there are 14 days: exactly 2 full weeks. Counting only Monday to Friday gives 10 business days. Adding 100 days to 2026-01-01 lands on 2026-04-11, a Saturday.
Day of the week
The weekday is computed from the same UTC timestamp, so it agrees with any calendar. This is handy for checking, say, whether a deadline falls on a weekend before you promise a date to someone.
Limitations
Business days are Monday to Friday only; public holidays are not subtracted, since they differ by country and region. Full months are whole calendar months: from March 15 to April 14 is zero full months, and to April 15 is one.