Trying to make sense of that "sentence."
I’ll explain:
Folks who assumed he drove when he took leave were mistaken — as to get the 9/18 postmark we know that mailing had to be after 5 p.m. That is because mail was picked up after 5 p.m. The FBI in its affidavit notes that the window for mailing was after 5 p.m. on 9/17. Thus, he could not have mailed while on leave during the day.