No specific date or period has been determined for the invention of the calendar, since it is believed that humans around the world have used different means to mark the cycles of nature throughout time. For instance, the ancient Egyptians, Greeks, Chinese, and Mayans all had various calendar systems based on natural observations of the Moon's cycles and seasonal changes. The Egyptians were the first to work out a formula for the solar year (based on the position of the Sun), which differs slightly from the lunar year (based on the phases of the Moon). This formula was eventually adopted by the Romans and was the forerunner to the modern calendar.
The first Roman calendar had ten months and utilized a number of blank days or "filler" months when necessary to stretch the year to its proper 365 days. This calendar ran on a four-year cycle and had far too many blanks, so it was easily abused for political purposes. For instance, to promote the length of their terms certain rulers added huge amounts of time to their calendar years. By the time Roman emperor Julius Caesar (100–44 B.C.) took power, the calendar year was so exaggerated that January fell in autumn. Caesar undertook a revision of the calendar and borrowed heavily from the ancient Egyptian methods, adding 90 days to the year 46 B.C.to allow the next year's spring to fall in March. Called the Julian calendar, it consisted of a year made up of 365 days, with one day added every fourth year (called leap year) to compensate for the fact that the solar year is actually 365.25 days. It had twelve months, each having either thirty or thirty-one days except for February, which had twenty-eight.
The calendar generally in use today is based on the Gregorian calendar. It was named after Pope Gregory XIII (1502–1585), head of the Roman Catholic Church (a Christian religion based in Rome, Italy), who revised the Julian calendar. While the Gregorian calendar is similar to the Julian, it was revised to bring the Christian celebration of Easter (the day Christ rose from the dead) in alignment with the vernal equinox (the first day of spring). It also dropped leap years for any century year not divisible by 400 in an effort to keep the solar calendar in line with the seasons. Although the Gregorian calendar is the one most prevalently used today by countries in Europe and North and South America, there are several kinds of calendars. These include the lunar Babylonia, Muslim, Chinese, Jewish, Coptic, Japanese, and Hindu calendars. Astronomers consult the Julian Day calendar, which allows differences between two dates to be calculated more easily than other calendars that have uneven months. It is based on the solar cycle, the lunar cycle, and the Roman indiction (fifteen-year) cycle. The starting date is January 1, 4713 B.C., and dates have been numbered consecutively since then, regardless of the various changes made in other calendars. Historians rely on the perpetual calendar, which contains the days of the week for both the Julian and Gregorian calendars, thus making it easier to place historic events on a uniform timeline.
Further Information: Meyer, Peter. Calendar Studies. [Online] Available http://serendipity.magnet.ch/hermetic/cal_stud.htm, November 8, 2000.