Watchtower: "If we were following a man undoubtedly it would be different with us; undoubtedly one human idea would contradict another and that which was regarded as light one or two or six years ago would be regarded as darkness now. But with God there is no variableness, neither shadow of turning, and so it is with truth; any knowledge or light coming from God must be like its author. A new view of truth never can contradict a former truth. "New light" never extinguishes "older light," but adds to it. If you were lighting up a building containing seven gas jets you would not extinguish one every time you lighted another, but would add one light to another and they would be in harmony and thus give increase of light: So it is with the light of truth; the true increase is by adding to, not by substituting one for another."
Someone may ask, "When did the Watchtower say this?"
My reply is, "If you believe the Watchtower is Jehovah's channel of communication, what does it matter? Didn't you just read what they said? Later light from Jehovah can't contradict what He has already revealed."
Just so you can check the reference, though, this was written in the Watchtower in February, 1881, p. 188.
Oh, but that was so long ago.
Evidently, later they got new light that said that new light CAN contradict old light after all.
And, of course, they speak for Jehovah because they say they speak for Jehovah, so there's no problem.