Time marches on, and with it man's parade of inventions for keeping it. The Hourglass, a clock shop, traces this evolution in timepieces, from the sublime to the ridiculous, with a clock to suit every mood and fancy. Dignified wooden grandfathers contrast with sleek seethrough versions. For a challange, there's one clock that will keep everyone but a new-math wizard guessing the correct (hint: it's based on the binary system). Another, certain to be a conversation piece, operates with metal marbles sliding down wooden shoots. There are also the predecessors of these mechanical marvels-sundials, saw clocks, water clocks, plastic replicas of a 15th-century wheel clock, and railroad regulators. The decorator seeking an unobtrusive clock will find framed nature photographs with the clock's face blending almost unnoticed into the landscape. For comic relief there are clocks attached to a miniature toilet seat and a toaster, and a numberless clock that asks "Who cares?" One digital clock lights up only at the clap of hands. Portable timepieces-pocket and wrist watches, tiny travel alarm clocks-and other novelties complete the selection.The Hourglass. Lakeforest Mall, Gaithersburg. 840-5707. CAPTION: Picture, no caption, By Bethann Thornburgh