NEC/safety for proximity of switch to a gas water heater

Can anyone tell me the guidelines (NEC or otherwise) about the proximity of a clock switch and a 60 gallon gas water heater? The wall-mounted switch is 5 feet from the floor, 13 inches to the top of the water tank, and about 4 feet from the burner at the bottom of the heater. If this is out of code I could move the switch, if necessary. This clock switch controls a small 20 watt outdoor transformer for landscape lighting. The mechanical clock switch is on a 20-amp rated GFCI circuit, enclosed in a plastic electrical box, manufactured in 2002, and a cover plate is properly installed. The gas water heater is about 10 years old, the same age as the house. Thanks.
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