How days are counted
The difference between two dates is the number of midnights between them: from Monday to Tuesday is one day. Tick “Include the end date” when both days count, for example for the length of a vacation or a hotel stay counted in days rather than nights.
Years, months and days are counted the way people usually do: from 31 January plus one month is the end of February. Working days are Monday to Friday; public holidays differ by country and are not taken into account.
Adding months
When you add months and the target month is shorter, the result moves to its last day: 31 January + 1 month = 28 February (29 in a leap year). Adding 30 days and adding one month are therefore different things.
Leap years
A year is a leap year if it is divisible by 4, except for years divisible by 100 that are not divisible by 400. So 2000 and 2028 are leap years, while 2100 is not. All dates use the Gregorian calendar.