In order to insert data into a date data type - you need a valid date or string that can be converted into a date.
If you are looking for random dates - then wouldn't you want random days also? Or are you looking for random months only? If random months only, then just pick a day - either the 1st or 15th would be my recommendation.
Jeffrey Williams
“We are all faced with a series of great opportunities brilliantly disguised as impossible situations.”
― Charles R. Swindoll
How to post questions to get better answers faster
Managing Transaction Logs