"Wasn't the gathering in episode 1 just to make the food for the Summer Sun Celebration? I imagine they'd want a good amount of help with that."
Applejack actually says specifically in "Applebuck Season" that "they were just here for the Apple Family reunion."
So, yeah, assuming the writers are trying to be consistent (doubtful) 100 moons = whatever time has elapsed between now and Twilight's arrival... which I doubt is 8 years. But it's a kids' show- who needs continuity!
True, they probably chose the term "moons" so that they could give some sense of regularity without giving a specific date.