Patchwork uses both the sync time and the user timestamp to decide viewing order. The oldest stamp wins. So if people were to start posting things as far in the past, no one would see them. Maybe based on that selection pressure, they might stop doing that. There is no advantage to posting things in the future except that syncing someone new would put them at the top of your feed for that moment. At which point maybe you would block them?

Sorry for the cheeky quips in there, but hopefully that answers the question in regards to patchwork.