PVRs are great for caching live events. Let it start the recording and leave it be for 15 or so minutes so that you've got enough "buffer" so that you won't catch up to those ads.
This approach is also good for sporting events where there might be a lot of nonsense and commentary that you don't necessarily want to see.
If it's a less interesting sport like soccer or most of what's on the Olympics, you can fast forward to the interesting parts.
Plus you don't have to worry about "starting on time". It's actually better if you don't.