Complex sentence: Jack talks this over with Liz and comes to realize that as "Some Like It Hot" was released in 1959, Jimmy Donaghy cannot have been his biological father because Jack was conceived in 1958, during the period Jimmy had been absent.
Split into simpler sentences: Jack talks this over with Liz, but comes to realize that "Some Like It Hot" was released in 1959. He realizes this means Jimmy Donaghy cannot have been his actual father as he was conceived in 1958, during the period Jimmy had been absent.