This is the DestinyPublicEvents.com official app. Its the MOST accurate timer available!
Most events have a 5 minute window of when they can happen. When the timers hits 0, this is the start of the window. When the timer hits -6, the window is over and you should move to the next location.
Events are typically a few minutes late, but its best to get there a little earlier and do some patrol missions. Sometimes events do not happen. Stick to green events for the best chance at getting an event.