That's how the 12th works out in my mind too. One detail that bugs me: if he went back home after Kmart, which I think he did, then why didn't he take the gifts out of the car and leave them in the house if he knew he was coming back to St. George before Christmas? Not a major issue, but it's what I would have done.
Maybe he left them in the car (the trunk?) because he thought he'd be driving them back to SLC, so there's no use bringing them back and forth?