A calendar is a system of measuring the passage of time. Early peoples used the lunar month--the interval between a complete sequence of phases of the moon--to calculate time. Because a lunar month contains about 29 1/2 days, a lunar calendar of 12 months results in a 354-day year. This is about 11 days less than the solar year--the length of time it takes the earth to make one complete orbit around the sun--of approximately 365 1/4 days. This discrepancy confounded calendar makers for thousands of years. Early Mesopotamian cultures used a lunar year to calculate time. In order to align their 354-day lunar year with the solar year, the Babylonians added a month (a procedure called intercalation) whenever their calendar got out of balance. Because intercalation was done irregularly, the Babylonian calendar became extremely complicated and confusing. To correct for this deficiency, the Babylonian lunisolar cycle was standardized in the fourth century B...