Basically, most world map events have a 'Did you do it?' flag. They activate when they are loaded in for the first time, and usually notified by an auto-save. If you then leave the area enough to load a different zone, go through a transition, or just plain go far enough away to unload them again, then that turns that flag into "nope, you didn't!", and the event goes to its default end state. Time passed has literally nothing to do with this, unfortunately.
