As the others have said if you just set up your recordings to start and stop on time, the DVR will acutally start the recording 30 seconds before the show, and end 90 seconds after the show, as long as the tuners are free. If the tuners need to be used before or after the show then it will just start or stop the recording on time and you won't get the extra time, but it will record the other shows.
The exception to this is if you have two shows set to record back to back on the same channel. If that happens the DVR is smart enought to realize it and it will record both of them using the same tuner, so both will get the extra recording time.
If you actually set your recordings to start early or end late manually, then it will always do so, and it will cancel other recordings as needed according to their priority in order to get those extra minutes.
So for most people it makes more sense to not tell it to record extra time, unless you know for sure that a certain show goes over pretty much every time. Otherwise you are better off just relying on the built in padding so you are less likely to miss other recordings.
Sometimes when I reflect back on all the beer I drink I feel ashamed. Then I look into the glass and think about the workers in the brewery and all of their hopes and dreams. If I didn’t drink this beer, they might be out of work and their dreams would be shattered. Then I say to myself, "It is better that I drink this beer and let their dreams come true than be selfish and worry about my liver."
-by Jack Handy