Sometimes it flies, sometimes it marches on, and sometimes it seems to stands still. Regardless of its speed, time has always been measured in some fashion, and the National Institute of Standards and Technology has an informative online presentation of the evolution of time measurement throughout the ages. Visitors will learn in the first three sections that calendars were the first timekeepers, then clocks, such as sundials (aka shadow clocks) and water clocks were invented, followed by spring-powered clocks, and finally the more accurate mechanical "pendulum" clock. Visitors interested in the most precise timekeeping will enjoy the sections on the "Atomic Age". "World Time Scales and Time Zones" rounds out the presentation, and explains the appearance of time zones, and the first cesium atomic frequency standard, which was built in England in 1955 in collaboration with the United States. The link entitled "Exhibits on Time", on the homepage, offers half a dozen sites that concern time, including the Smithsonian exhibit, "The Quartz Watch".