I freeze butter all the time - I buy it in bulk from Costco, freeze it immediately, and stock my refrigerator with just enough to last me a week or two. If memory serves me correctly, Rose freezes her for up to a year.
As for the expiration date issue - salt acts as a preservative, so salted butter will last longer in the refrigerator than unsalted butter, which is reflected in the respective expiration dates. I wish I could tell you how long beyond the expiration date your refrigerated butter can be used. I guess that depends on how cold your refrigerator is - your nose should let you know if the butter has gone rancid.