You declare it as the first line after "function getNextDay() : date {", then it is glaringly obvious that is a date variable that will (eventually) contain tomorrow's date, and will be returned by the function.

However, I would only use "var" if it's initialized in the same statement. It prevents Smurf code, and the compiler knows the type straight away.

Given a small and clean context, variable names don't need to be specific.