According to Kevin Parks, it just makes life easier for everyone if they just pretend that it's always the current year, no matter how much time ACTUALLY passes.... that was after a friend of mine actually brought it up to him at a convention and asked about it. That was his answer. Kevin is just an assistant director though - I think if you really wanted an answer, you'd have to ask the writers or the producers...or the showrunner, both Gamble and Carver are guilty of it. Kripke was actually the best for time working fairly well and consistently when he was in charge.