I'm always confused by the times for which college football games are scheduled. If a game (say, like the Oregon/Ohio State game tonight) has a start time of 6:30 PM, does that mean that the kickoff occurs, quite literally, at 6:30 PM? Or, does this 6:30 start time signal the commencement of "pregame" procedures, such as teams running out on the field, coin toss, etc?
Start time is the estimated kickoff time. Back in the day some tickets would actually say EST Kickoff Time 2:00 PM. However most tickets don't include times anymore since game times are so often changed for TV networks. +1 for anyone that can find an old ticket with Kickoff Time printed because they definitely existed.
As for NFL and major colleges you will find that their kickoff time is usually right at the estimated start time. They want that kickoff right at the slotted time to get the viewer hooked in. In the NFL a game is behind if the kickoff happens at 12:03 for a noon game. This does happen and might be a variety of reasons - malfunction at stadium, ref needs to hit the bathroom, whatever - but most games do start right at noon (within a minute) if scheduled for that.
College games are pretty much the same way although there is more variance for issues to arise and the games will more commonly start a couple minutes past estimated kickoff time (which is the same as start time). I have refereed small college games and we are told that captains meeting is 10 minutes before start time and they want our games starting right on time. For smaller colleges/high school the captains actually might have a bunch of questions, which we answer and we might start a few minutes late. I have also had both teams run out, no questions, run back, and expect the kickoff right away. Which we try to slow things down but have started a minute or two early.
For major colleges/NFL they will never start early because TV networks can't support this. They get cues on the earliest they can kickoff. I have been to plenty of college games where the kickoff teams have stood waiting on the field for 3-4 minutes. Everyone is wondering the delay but its the TV networks.
The exception of this rule is Superbowl, Monday/Sunday Night Football and college bowl games. These are big events and the estimated times are a pipe dream - they are close for Sunday/Monday Night Football but still may be 5-10 minutes late if they have breaking news they want to cover. That is when they want you to start watching and kickoff may be 10-20 minutes away. They can really only do it if there isn't another competing game you can switch to.
So to answer your question the start time is the estimated kickoff time. There are no hard rules governing this though other than TV networks. Referees at all levels have specific time slots for the toss and captains meeting which is always before the start time and allots enough time in most cases to start when people are expecting it. You can always see the whole game if you tune in at that time but you might be a little early for major events.