My thought is that the tape deck doesn't have a memory backup battery that would run the clock during a power outage (unplug). If it were installed in a car, then it would have a wire (probably labeled "clock") connected to continuous power that would hold settings for the clock. When you connect it to the computer power supply you loose all power when you unplug so you loose the clock. To keep that from happening you'll have to keep it plugged in. That's not what I'd do since it's a DIY affair. I'd just ignore the clock.