This is probably the best argument to refute their claim that I've yet heard.
THAT'S THE THING! How can people cite the bible say when they say that the world is ending on X day because if you decode it upside down after you've poured water on it and then plug in the leftover letters to x algorithm when your drunk, it says so!
When in very plain language it says that "any day you say it will happen, it won't."
I can only answer this question bluntly, and the blunt fact of the matter is that interpretations of the Bible that require going beyond the plain revelation of God's Word are done for either 1) a presupposed goal that makes every text fit that goal regardless of context or any other accepted interpretation method; and/or 2) profit.
Avoiding these errors is incredibly important to me. As a pastor I must endeavor to preach what God has said, not what I want to say or what I want His Word to say. Likewise, every errant move of the church from the days of the Apostles has came precisely because man twisted the Scripture to fit himself, rather than submitting to God in accordance to His Word. God does not take this lightly (Isaiah 8:20).