You can launch a 2019 model on December 31st 2019 if you wanted. But at 12:01am January 1st, the vehicles coming off the line VIN's better reflect it's a 2020 model.
If you're right and the Ranger is a 2019, and it's not on sale before the end of this year, then 2019 is going to be a short model run. Should just launch it as a 2020 then.