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Th zener diode will clamp or limit the voltage to 12V. The resistor ir required to limit the current to the zener when the battery voltage starts to go above 12V, which is the normal condition when the engine is running and the alternator is working. Without the resistor, the zener would burn out connected directly to the battery. If the clock was drawing a continuous current (I don't think it does) you could lower the voltage with a series resistor. That voltage would still vary with the battery voltage it would just be lower. If the clock only draws current when it winds the spring you'll measure pretty close to the battery voltage regardless of the resistor value.
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