This problem can happen if you disconnect the battery or if the battery get disconnected by it. You can tell if this is what happened because the year will say 1970 or so. The best way to solve this is to connect to WiFi and sync to the correct time. Then, switch airplane mode on/off a few times, remove the SIM card, and power off your iPhone. Reinsert your SIM and that should do it! Good luck with that.