If we were to delete adds when they were a month old we would be deleting adds every day. That woud mean going through and checking the date on adds every day, no thanks.
Yes if you post on the 31 its gone the next day. So now that you know that you can do one of two things. Save you add as a word file and copy in again on the 1st or, don't post adds at the end of the month. The membership now knows how it is going to do so they can make an informed desicion on when to post.