Killing the player in a trigger zone.

I need to restricts player’s movement by killing him if he enters the trigger zone and stays there for too long (20 secs maybe). I know how to do the killing script and assign it to a trigger zone but I have no idea how to do the 20 second delay. The thing is that it has to restart each time a player leaves the zone and then enters it again. And right beore you die I need to play a sound.

function OnTriggerEnter (other : Collider) {
Destroy(other.gameObject);
}