Count the days between two dates, or add time to one

Pick a mode and the result updates as soon as both dates are valid. "Days between dates" gives the total days, a calendar span in years, months and days, the same gap in weeks and days, and the equivalent hours. "Add / subtract time" takes a start date plus any number of years, months and days in either direction and returns the resulting date with its day of the week. "Working days" counts Monday to Friday between two dates, with both ends included, and shows the weekend days it skipped.

Month lengths and leap years come from the calendar rather than from an assumed 30-day month, so the span between two dates borrows days from the month that actually precedes the end date. Enter the dates in either order: if the end date falls before the start, the calculator says so and shows the absolute span. When adding months to the 29th, 30th or 31st and the target month is shorter, the extra days roll into the following month rather than being clipped to the month end.

The working-days count excludes only Saturdays and Sundays. It does not know about public holidays, which vary by country and year, so subtract those yourself for deadline and notice-period calculations. Dates are interpreted in your local timezone and nothing is sent anywhere, so it works offline once the page has loaded.