@kynnath that's a very great observation and theory... I'm so amazed
Also Willem said he went back to interrogate them but there wasn't really much further interrogation when he knew it threatened fmc saying that "theres no clue that can lead to mastermind" that's really a weak one.
But yeah as what you've said, we don't know if that's the intention of the author since as willem' pov was seen or maybe someone heard their convo for the date as ordered by someone (spy: coachman or guard) or maybe author just did it without a mere thought of where did the mastermind got the source of their date and time.