4000 BC

4000 BC is the earliest year that the game currently seems to be programmed to be able handle, essentially making it the beginning of time. It seems that it is also the default year that gets fallen back to if you try to time travel to an invalid date. In-game, it currently appears as nearly identical to 2387 BC, likely because that is the earliest date the player is intended to see in the main story.

One method of reaching this year without cheats is the unknown destination chip will take you here instead of 9999 if you have the lighting rod on your car, though this is most likely a bug.