After seeing the way they SCREWED Admiral Flynn, I would not be surprised to see this exchange:
FBI INTERVIEWER: Mr. Papadopoulos, can you tell us the date you met with Mr. Halper?
PAPADOPOULOS: Well...let's see, that was sometime when I was in London...I returned to the States on the fifteenth, and it was a few days before...I think September 12th.
FBI INTERVIEWER: I see. When you got back to the States...
In the back room, they were probably looking at illegally obtained transcripts of the meeting from a wire-carrying Halper, and said "Aha! We got his ass! The meeting was September 13th, not September 12th!" with a lot of ensuing high fives and jollity.
Would not be surprised at all.
Read the information they filed against him to get his plea. It’s on the web.