stormdog | Units of Frequency Smaller than 1 Hertz?

Also, I was thinking about the functioning of the Kenosha bus system in terms of its clock speed in hertz. If buses leave the central station every half hour, the system is running at one cycle per half-hour. Megahertz and gigahertz are used frequently as measurements, but I've never seen the related terms for measurements smaller than one cycle per second. I did math in my head while riding and decided that Kenosha's buses have a clock speed of 1/1800 hertz. At first I thought that was 1.8 milihertz, but that's not right, is it? It would be a smaller value than 1 milihertz, because 1/2000 hertz would be .5 milihertz. I ought to be able to figure this out, but can a math person tell me how to calculate this?

Does anybody use such terminology? It doesn't seem like it would have wide application, but it's kind of fun.