I need to bring to your attention that this is still happening.
Today, fortunately, I had my campaign ready to go, unlike before, so no damage was caused.
However, today I set the sending time to 12:20 p.m., giving myself plenty of time to write my second newsletter before sending out either of them. (I sometimes catch typos and other mistakes this way.)
I double checked the date and time. They were correct, 07/2020, 12:20 p.m.
I clicked on the final submit button, taking me back to my dashboard, where the campaign read PENDING-SENDING, and the time was correct. It did not appear to be sending.
I copied it to use it as a template to write the newsletter for the other half of my subscribers (I have two lists).
By the time I finished writing the second newsletter, when I returned to the Dashboard, I discovered the first newsletter had already been sent BY MISTAKE. The date and time had somehow changed to the time I started writing it at 9:21 a.m.