My time and date keep changing!

everytime i turn on my computer after i've shut it down, the time's always changed to a date "before 2008". then i go and change the time and date in my System Preferences and lock it, but if i am to shut down my computer and turn it on, the time's changed again. this is very frustrating and deters me from shutting down my computer, which is something that i like to do.