Where does it say that in the Bible? Deuteronomy 18:22 says that the test of whether a prophet is true or false is if a prophecy comes to pass or not, regardless of specific days/dates.
FWICS, the only one who lays claim to 100 percent precision is the Father, and specific days are His secret alone (Matthew 23:46, Mark 13:32).
But the prophet who presumes to speak a word in my name that I have not commanded him to speak, or who speaks in the name of other gods, that same prophet shall die.’ And if you say in your heart, ‘How may we know the word that the Lord has not spoken?’— when a prophet speaks in the name of the Lord, if the word does not come to pass or come true, that is a word that the Lord has not spoken; the prophet has spoken it presumptuously. You need not be afraid of him.
Oh boy, here we go with another person cherrypicking that one verse and using it as "THE STANDARD". To me that just shows ignorance and no real understanding of judging prophets and prophecy.
Isaiah was one of the greatest prophets in the old Testament, the prophet that gave the most prophecies about the coming Messiah, the prophet that the Messiah himself quoted.
Isaiah prophesied to King Hezekiah "Thus says the Lord: Set your house in order, for you shall die, you shall not recover.". No conditionality to that word. Hezekiah DID NOT DIE, the prophecy DID NOT come to pass. Should Isaiah be stoned to death? Was he a false prohet?
I call this the "Cherry Picker's Isaiah problem".