This site contains a great deal of information about the history of timekeeping. It mainly emphasizes the historical and cultural reasons for developing ways to keep time, rather than explaining the scientific processes behind the methods. Readers get a good perspective on the relationship between science and the needs of society, such as why calendars, clocks, and world time zones were developed. Regarding the more technical topics of how quartz crystal, pendulum, and atomic clocks work, this site offers very brief overviews.
As for general interest, this site is really a readers' digest account of historical/scientific facts about Earth time. There are no graphics or illustrations of the information, just text and a few sketches. There is, however, a lengthy bibliography for those interested in more depth and detail about timekeeping. This site could be valuable to students planning a career in physics and for those already in the technical field. There is a link to the NIST (National Institute of Standards of Technology) Physics Laboratory Page, which offers information on research internships, seminars, physical data, and hundreds of other resources associated with the NIST.
This site would be suitable for high-school or college physics and astronomy teachers as a supplementary source of information. Teachers of advanced middle-school science classes may also find it useful. There are no lessons or lesson plan ideas here, but motivated teachers could find ideas for creating their own lessons.