Why would you have to reset the date and time every time you turn on the car and how can you fix this?

If it is on the radio and most likely an aftermarket system they wired it wrong. There should be a wire that has constant power from the battery to keep up on the time. If its in the computer system I am not sure. Probably similar though like a bad wire so its not getting that power from the battery all the time to keep it accurate. I had the same problem, phoned the dealership, who suggested that my battery was shot and not holding a sufficient charge to maintain the clock. I replaced the battery and my problem was solved.