I'm sure I'm not the only fan writer in Parks and Rec fandom to be utterly baffled/frustrated/annoyed when trying to figure out the timeline of the show since Ben and Chris arrived. The whole mess derives from the fact that the makers of the show thought they'd be airing in the Autumn of 2010, but were bumped back to mid-season instead.
In season 4, the writers of Parks and Rec have made a few valiant attempts to retcon the show timeline since Ben and Chris arrived into making sense, and they've come close, but not quite managed. So I've made an attempt (for purposes of personal obsession and for keeping the timelines of my fic accurate, damn it!) to make sense of it all. I started this as a plotting tool just to help my own fics, but I think other writers might find it useful. Other folks might also have input on how to amend/improve what I've come up with, and I'd love to hear your input.
I think this could be helpful to Leslie/Ben writers, and to Parks and Rec writers in general.
My obsessive timeline thoughts here