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Not sure what country your in (since in the US clock shift doesn't happen until Nov 2nd), but these answers should hold for the developed world as a whole...

for the PC... MS/Mac/Linux programmers actually put in the dates it changes along with localization settings to make sure it's correct for your country. When your computer hits the proper time it adjusts the clock. Your computer knows what time it is based on an internal clock, which is powered by a small battery when the system is off... leave a computer unplugged for a few years (few months on really old computers) and the clock will reset. Most OS's now do an internet time sync with a clock server.

The DVD player does a very similar thing... your national government broadcasts a signal that tells any device listening what time it is, the signals are adjusted for DST so the clock doesn't have to worry about it at all.

Also if your cable box connects to the cable provider for time sync, and if your DVD player hooks into the cable if can get a time code from that as well.

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