And Cordelia, it isn't "just the dates" the "Slave" got wrong. It was, and this is HUGE, what they said would happen on those dates that they got wrong.
It wasn't just a matter of building up expectation for "something, but we don't know quite what." It was saying that the Bible confirmed absolutely that such-and-such would happen in such-and-such a year. That is called prophecy. Your dad doesn't seem to grasp that it wasn't just a matter of expecting certain dates, it was a matter of foretelling events in Jehovah's name.
Deuteronomy 18:20-22 says: “‘However, the prophet who presumes to speak in my name a word that I have not commanded him to speak or who speaks in the name of other gods, that prophet must die. And in case you should say in your heart: “How shall we know the word that Jehovah has not spoken?” when the prophet speaks in the name of Jehovah and the word does not occur or come true, that is the word that Jehovah did not speak. With presumptuousness the prophet spoke it. You must not get frightened at him.’
Was Jehovah using the "Slave?" Was His spirit directing them? According to them, yes. Did they speak of future events in Jehovah's name? YES! Did those events come true? No.
This is the only definition of false prophet in the Bible. If your dad wants to stick with dates, that's fine. When someone spoke in God's name and the word failed to come true, could they just say, "Woops! You were overeager in your expectations. Try to keep your anticipation under control next time," and escape accusation as false prophets? No.
What did Jesus tell us directly to be warned against and watch out for?
Matthew 7:15-20 “Be on the watch for the false prophets that come to YOU in sheep’s covering, but inside they are ravenous wolves. By their fruits YOU will recognize them. Never do people gather grapes from thorns or figs from thistles, do they? Likewise every good tree produces fine fruit, but every rotten tree produces worthless fruit; a good tree cannot bear worthless fruit, neither can a rotten tree produce fine fruit. Every tree not producing fine fruit gets cut down and thrown into the fire. Really, then, by their fruits YOU will recognize those [men].
Jesus was talking to Jews. By what fruits would they recognize a false prophet? In context, it is clear that the fruits Jesus was talking about is the results of their prophecy. That is how Jews figured out who false prophets were. Jesus warned against this Organization.
For what it is worth, it is a simple line of reasoning and maybe your dad can get it. Especially if he already believes they got dates wrong.
AuldSoul