PublishPress Can Sometimes Change Publishing Dates
If you un-publish and then re-publish a post, the PublishPress plugin will reset the publishing date. When a post is published, the PublishPress plugin will update the post's publishing date to the day and time of publication.
This is a little different from the normal WordPress approach, but we think it makes sense for most (if not all) situations.
If this doesn't fit your needs and you do want to choose a different publishing date, please choose the other date after publishing your content.