the Zone-Event basically triggers every 18 min with a delay of 0-3 min in between. check the message in chat that the event resets +18 to 21 min = spawn of the dragon^^
You 100% sure? I have check that a lot of times, and for what I can say the dragon doesn't spawn with the event. The event run for 18 min and have 2 min pause, so always starts at 11, 31 and 51 min (+/- 1 min lets say for server delay on real times). I have check it again just now to be sure, and no dragon. Even if someone actually runs the event and finish it, that only extends the pause, the start times are the same.