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.C. to a calendar of 19 years and 235 months. The ancient Egyptians developed a solar calendar based on their observations of the annual appearance of the Dog Star, Sirius, and the nearly simultaneous flooding of the Nile. Sometime between 4000 and 3000 B.C. the Egyptians adopted a 365-day year, with 12 months of 30 days each plus five extra end-of-the-year feast days. The ancient Hebrews used a lunar calendar, with 12 months of alternating 30 and 29 days, adjusted with irregular intercalations. After A.D. 300, the Hebrew calendar adopted a fixed intercalation system of adding seven extra months during each 19-year cycle. The Chinese calendar, allegedly invented in 2637 B.C. by the legendary emperor Huangdi, contained 12 lunar months with the same seven month-19 year intercalation schedule. The sequence of years in the Chinese calendar is traced in cycles of 60, with each year in the cycle designated by two terms, one of which is the name of one of 12 animals that recur in order. In Mesomerica, the Maya Indians, who were excellent astronomers, developed a very accurate calendar. It consisted of 18 months of 20 days each, for a 360-day year, with five extra days (considered to be unlucky) added. The Mayans also observed a parallel 260-day cycle, which was probably used in religious ritual. The ancient Romans, like the Greeks before them, at first based their calendar on the lunar month. This 10-month year was supposedly established by Romulus, the legendary founder of Rome. The months of January and February were added, according to tradition, by the ruler Numa Pompilius around 700 B.C. This 355-day year was supposed to be periodically adjusted by insertion of an extra month, but the intercalation was erratic.
By 46 B.C. it was evident to Julius Caesar (100-44 B.C.) that the calendar needed revision. He called on the Greek astronomer Sosigenes, whose suggestions Caesar adopted. The new Roman calendar year was divided into 12 months of 30 and 31 days, except that February had 29 days; every four years an extra day was added to the year to account for the 365 1/4-day solar year. (When the Roman emperor Augustus (63 B.C. -14 A.D.) renamed one of the months after himself, he took a day away from February and added it to August so that his month would be as long as July, the month named for Julius Caesar.) This revision, called the Julian calendar, remained in use for 1,600 years. Although the Julian calendar was very accurate, it was 11 minutes and 14 seconds too long per year. By 1582 the discrepancy had caused the spring equinox to occur 10 days too early. Pope Gregory XIII (1502-1585) reformed the calendar again so it would conform to the now-known actual solar year. He ordained that 10 full days be cut from the current year, so that October 4, 1582, was followed by October 15, 1582. He also devised a system whereby 3.12 days are dropped every 4 centuries. The new Gregorian calendar was extremely accurate and gradually was adopted worldwide, with Protestant countries lagging behind Roman Catholic nations. England waited until 1752 to switch to the Gregorian calendar. Because the year 1700 had been a leap year according to the Julian calendar, England now had to subtract 11 days from its calendar in order to adopt the new system. The disappearance of thse 11 days led to riots in that country. China adopted the Gregorian calendar in 1912, the Soviet Union in 1918. Many Islamic countries continue to use the 354-day Muslim calendar, which has 12 lunar months of 29 and 30 days each, with an extra day added in 11 years of a 30-year cycle. The Bahai calendar, introduced in 1844 by Seyyèd Ali Muhammad, operates on a 19-week year within a 361-year cycle. The Hebrew, Hindu, and Bahai calendars remain in religious use. With the growth of computerization in the late twentieth century, increasing amounts of calendrically based data began to be stored in computer software fixed-file formats. The inaccurate programming of the software binary dating systems led to the potential misreporting of the digit "00" in the year "2000" as "1900." If left uncorrected, at the turn of the millennium this discrepancy would result in a massive miscalculation of records worldwide. In addition, the Gregorian rule that centennial years divisible by 400 are leap years raised the possibility that two-digit binary systems not recognizing the year 2000 would be thrown one day off reckoning after February 28, 2000. As of 1998, businesses and governments across the world were working together to bring about a year 2000 software conversion. this discrepancy would result in a massive miscalculation
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