I honestly don't know about the "Hangup" response, I literally just learned of the hwclock command yesterday, however, having the time be screwy after reboot is a tell-tale sign of a dead/dying mobo battery. Forgive me if I talking beneath you, but this is how the computer keeps it date (along with a few other settings). Oversimplified, it's all stored in a RAM-like chip that gets the power to function off of a little 3v battery on your motherboard.
If the motherboard battery gets low on juice and can't quite put out enough power for the chip to keep its settings, then the settings will start to slip, such as your HW date setting. If you crack open your chassis (WARNING: This may void a warranty of some sort) you should be able to see a little round battery (like what would be in a watch) in plain view. Replace that and reset your settings and chances are your HW clock will be good. Best of luck