So three more pieces of information. From the YVR website. "Please Note: US CBP is operational between the hours of 4:00 AM - 8:30 PM. Passengers who are departing on flights outside of this time should contact their airline for more check-in information."
So I called and thoroughly confused the agent. She said that if I got to YVR before 8:30 I would have the option of going through preclearance. If I didn't get preclearance then I would have to go through customs in JFK. She said they would sort people in JFK by whether or not they went through customs in Canada. I tried to explain that either the flight goes through preclearance or not and they are not going to mix people who went through preclearance with people who didn't. It seems like 1356 has been departing out of gate D66, which isn't in the "US" part of the airport. I've tweeted @DeltaAssist so we'll see if they can iron it out.
The whole reason I care is it determines what time I need to get to the airport. If I need to go through preclearance I should probably be there at like 8 just in case something happens. I don't want to show up at 8:31 and get stuck in Canada. That means I have to be at the airport 2:30 (or at least 2:01) minutes ahead which is a lot of time to waste at an airport. I know YVR is nice but I rather have more time in Vancouver.