Upon defuse start BarTime is fired with the time (in seconds) it takes to "arm" the bomb. When done (break or arming sequence finished) the event is fired with "0". The BarTime event offers no info about the success of the action. You must use the logevent to check if it was successfull.
BarTime is however inaccurate as other plugins use this event to "delay" other actions. Bomb plant usually takes 5 seconds. If this event is "shared" then the other plugins musn't use the 5 sec flag in their actions. Thus all you have to do is check the time fired with the event as a delay of "5" would always mean a bomb planting. Still if you have doubts about it then is better you don't use this event.