All you need to do is set the "detonation time" and activate the bomb. At the specified time, timebomb will shut your system down and (if possible) turn off the power. It's all just a few clicks away. No need for confirming the shutdown... no need to be around at all. Just take a look at the screenshots to get a look at this small but useable tool.

And yes, it's a timed shutdown tool that works on XP systems too!