Stopping the Clock

[donaldm4]

A friend of mine recently asked me a question about stopping the clock. I always thought that in the Pro game that when a player went out of bounds that the clock stopped. My friend pointed out during the early part of the first quarter of a game we were watching, that after a receiver had gone out of bounds, that the clock continued to run. Number 1 when did this rule change and Number 2 at what point in the game does the clock stop after a player has gone out of bounds?

Thanks