Also we don't want to use the Time command, since unless I'm mistaken, the Time command still resets to 0 whenever you load a savepoint in ArmA (like in OFP).
For this reason alone it's better to either spawn a little script that uses the internal _time command (which supposedly works even past save/loads), or to use the min/max/med entries on triggers, which also supposedly work past save/loads.
This trigger might work:
Activation: Anybody
Countdown: 480/480/480
Condition: true
On Activation: if (player in thisList) then {"2" objStatus "DONE" explo = false} else {"2" objStatus "Failed" explo = true};
In this way, the countdown will start right on mission start, and when it ends it does a check to see if the player is present. If he is, then everything is good, if he isn't, things explode. If there's something he needs to do (say, kill a terrorist), you can just add that to the condition line: true && alive terrorist1. That way, once the terrorist is dead, the trigger will never activate (and you can add the "2" objstatus "done" stuff in another trigger).
Hope that works.
Wolfrug out.