Scholastic
Scholastic Internet Field Trip: Calendars & Time Measure
Study the math and history involved in the development of the 12 month calendar. There are lots of links to informative time and calendar websites to assist you.
University of St. Andrews (UK)
University of St. Andrews: Mayan Mathematics
After a historical introduction, this site from the University of St. Andrews provides information about the mathematics of the Maya Civilization and shows just how advanced they were.
NumberNut
Number Nut: Calendar Origins
History and mathematics merge in this lesson that explores the origin of the calendar. Learn how it all got started in this detailed lesson that includes a simple months of the year game and a more challenging time conversion game. Both...