Thanks to you who've tried to help me think though this. Looking at different movie players didn't make any difference. Some were slideshows and they made fine mp4s. The actual videos I was editing came from Zoom. Looking more closely I saw that all my Zoom videos had the date stamp. Looking into Zoom settings I realized there was a setting to put date stamps on the videos. Maybe someone else reading this will learn from my experience.
Thanks for being there.