Why date math is harder than it looks
Months have 28, 29, 30 or 31 days, leap years add a day every four years (except most century years), and daylight-saving changes can make a naive "divide by 24 hours" calculation come out one day short. These tools do all arithmetic on calendar dates rather than on clock time, so the answers match what you would get by counting on a paper calendar.
Common questions
Do the calculators include the end date? By default, "days between" counts the number of midnights you cross, which is what most people mean by "how many days until". Where it matters, a checkbox lets you include the end date as well.
Which calendar is used? The proleptic Gregorian calendar, the calendar used worldwide for civil purposes today.