Your TraXX product contains a battery-backed real-time clock that is able to keep the system time even when power is removed from the unit. If your TraXX unit is older and has spent much of its life unplugged, it's possible that this battery could be dead. Replacing this battery will restore the units ability to keep time when powered off.
The battery is a CR1216 button cell battery and is user replaceable. To replace the battery, remove the top center screw from the front panel of the TraXX unit, and lift up on the cover. The battery is located near the center of the main TraXX circuit board.