Kimmy says the prom photo of Steve and DJ was a surprise. That photo was from Steve's prom, which wasn't a surprise. The surprise was when Steve showed up to take DJ to her prom- when her hair was really short, and she wore a gold dress.
In Season 3, Episode 17, "Fullers in a Fog," Danny comments on how his little girl looks great, and she replies, "You realize I'm 40 with three kids." DJ must have been rounding up as she turns 40 in this episode. This, along with her 39th birthday back in Season 2, shows that the seasons don't represent years. (Even though it is mentioned repeatedly this season that Kimmy and Stephanie have lived with DJ for five years.)