Beat me to it, on the outbound its OK I did it (didnt go for the UP wasnt on a fare that would have allowed it)

For the return the problem is that theres a flight SFO-JFK before the 870 leaves and after you arrive. So I broke my return up and saved myself from having to hang in SFO for an extra 1 1/2 hrs. Id suggest if still possible to book the return that way to do so and forget about the UP SFO-JFK. I needed to do it via Multi-city to get it to pop up (I havent checked UAs flight times on this run since I flew last May so things might have changed)