The key here is to NOT hit the 'Publish' button on a new post!
Hit the '+' button to start creating a new post, and then navigate to the Meta Data section to update the 'Publish Date' of your choosing.
If you hit 'Publish', the app will not allow you to change this date. So feel free to continue editing the post and let the app do the saving.
Hitting the 'Publish' button after setting a Publish Date in the future will override the date you've chosen.