If you want a device you could do this with SmartThings, but it would actually be a bit easier to do it with the free IFTTT service. You don’t need any extra devices at all. Harmony and Phillips already have IFT TT channels, and you can also use your phone’s presence.
Yonomi is a free app which can also do the same thing. Some people find it it runs somewhat faster than IFTTT does although it has fewer device partners. It should work for the case you described, though.
Whichever way you do it, the first step is to have a harmony activity which does exactly what you want. Because all that SmartThings or IFTTT or Yonomi will do is trigger that harmony activity.
In your case, I would think the simplest thing would be just to define your HTPC as an “always on” device in harmony. Once you do that, turning off the activity does not turn off that particular device. That’s what people do for cable set top boxes so that their video recordings will still take place even when the rest of the home entertainment system is turned off.
So there are lots of ways to automatically schedule the same thing once you’ve created a harmony activity that give you the results you want.