Used with
permission of Maiken Naylor, University at Buffalo, Buffalo, NY, USA,
http://ublib.buffalo.edu/libraries/units/sel/exhibits/stamps
Since prehistoric times, calendars were devised to keep track of time - days, seasons, years. The Julian calendar introduced by Julius Caesar, based on the solar year and containing leap year, was succeeded by the Gregorian calendar, after minute discrepancies had accumulated over the centuries. Early Central American calendars had 18 months of 20 days, plus five days of bad luck. An Aztec calendar stone was the symbol of the 1968 Olympics in Mexico City.