Luke 1:18
Luke 1:1818“How can I be sure of this?” Zechariah asked the angel. “I am an old man, and my wife is well along in years.”
is rarely cited in Watchtower publications. Zechariah's question to Gabriel ("How can I be sure of this?") is read as honest puzzlement rather than as unbelief, and Gabriel's response in v. 20 (striking him mute) is treated as a corrective rather than a punishment. The verse does not interact directly with the birthday-ban argument; it appears here as a counter-example to the rs hermeneutic.
Use to expose the inconsistency in the Watchtower's reasoning about narrative-as-prescription. The rs book's birthday ban derives a universal prohibition from two pagan-tyrant birthday narratives (Pharaoh, Herod). Apply the same hermeneutic consistently: Zechariah asked God a question, was struck mute as a consequence, so therefore asking God questions is forbidden; Jesus cursed the only fig tree named in detail, so therefore fig trees are evil; David wrote a letter to have Uriah killed, so therefore writing letters is evil. The reductio ad absurdum is the point — the rs hermeneutic, applied consistently, produces results that no JW would defend. The hermeneutic itself is the problem, not the application of it to birthdays specifically.