Eastern Standard Time - S4-E18
Plot hole: When Jen introduces Joey to her father in his office, she calls her "Joey Potter from Capeside". But later when they are at dinner, Jen's father suddenly refers to Joey as "Josephine" which seems strange as he wasn't told that was her legal name, and only those who know her well would know that. Joey is not necessarily a nickname only for Josephine, so it wasn't just a logical deduction.
Answer: It's only explained once in the series. Jack says to Andie "I tried to start kindergarten without you", meaning Andie was smart enough to start school early.