Date range calculator
A date range from 1 to 31 January is 31 days if both ends are included and 30 if you count the gap. Ranges are almost always inclusive; a booking from Monday to Friday occupies five days, not four.
How to measure a date range
Hotel bookings are the classic exception and cause endless confusion. A stay from Monday to Friday is four nights and five days, because the last date is a checkout rather than an occupied day. Car hire usually counts the same way. Almost everything else. Leave, project ranges, subscriptions: counts both ends. When a form asks for a range, the safe move is to give the dates rather than the count.
Questions
Usually yes for a period you occupy. Hotel stays are the notable exception, where the last date is a checkout.
Thirty-one inclusively, thirty as a gap.
Inclusive. Monday to Friday off is five days of leave.
Nights, which is the exclusive count. Monday to Friday is four nights.
No, the range length is the same either way round.