End user calendar validation - what and how to test?

If you wish to validate if a calculator application works, you can do some quick testing - like dividing with 0 (crash testing) or precision (floating points).

What sort of test would you do if you wish to validate if a calendar application that is being used to schedule reminders works as intended?

One thing I could think of would be to test

• Leap year
• monthly reoccurence when set to the 31. day of the month

What else would be good to test for? Are there any "standard" testing of such?

(Bonus info: I can't look at the source code, but need to know that the application is reliable)