I believe your words is the neatest one, no joke this time, I am serious. If you really like some words to cover your expertise, here is my dirty sentence below.
The program defines a Date class, it provides some functions to manipulate date, such as output a date in different formats. Unit test are also included in the program.
I don't need a default in it. It is already stated previously in the code.
Just because you don't need a default case doesn't mean you shouldn't use one. It doesn't hurt to have a default case that simply outputs a message saying that something went wrong with your switch statement.