The clock on San Francisco's Ferry Building marks the hour, and the surrounding area at the base of Market Street is abuzz with noise. Traffic roars along the Embarcadero. Vendors chat with tourists.

Tours of Big Ben are run like clockwork, for good reason: They have to be on time to catch the resounding climax. Terry Eiss, guide to the world's most famous clock, is a stickler for timekeeping as ...