History of the Calendar
Background. Taking the first chapter of the Book of Genesis as our starting point we find that the heavens were created first then the earth on the first day [v.1] while the sun, moon and stars were created on the fourth day [v. 14-19]. The sun and moon are described as “two great lights” [v. 16] while their given purpose is to, “divide the day from the night” and to be for, “signs and seasons, for days and years” [v. 14]. Finally, God saw everything He had made and declared it to be “very good” [v. 31]. Reasonably, we might expect that the first calendar would have been an easy one to reckon and very accurate. God carried out His entire creation during the first six days of the first week, including creation of Adam and Eve on the first Friday afternoon; He rested on the following day thus setting the familiar pattern for our 7-day week [Genesis 2:2]. As far as is known and with one dismal exception, every nation has always had a 7-day week. The exception arose from the French Revolution of 1789 and the introduction of the metric system with a ten-day week; the longer week was a short-lived failure. The days were always set by the time taken from one sun-rise to the next – it is easier to observe the moment of sunrise rather than to observe the sunset while the length of each day plus night is always the same, i.e. 24 hours. The lunations of the moon, from one new moon to the next, provided the divisions into months. When first given in Genesis 1 this calendar cycle, easily read from the sun and the moon, was especially important for farmers to plant and harvest their crops. Later, it would be important for religious observance i.e. the full moon to celebrate the Passover and later the related day to celebrate Easter in the Christian calendar.
The year was the time taken for the earth to make one complete orbit around the sun. However, scripture does not say how many days there were in each year of that pre-flood world but there is reason to believe that it may have been 360 rather than 365¼ days familiar to us. The earliest calendars available to historians today are those of the Babylonians who, according to Scripture, were the first nation following the Genesis Flood [Genesis chap. 11]. The Babylonian calendar contained 360 days and its legacy remains for us today as the circle divided into 360 degrees i.e. one degree per day as the earth makes its circuit around the sun. Of course, we know that in our post-flood world the year has always had 365¼ days. The Babylonians were therefore short of the year by 5¼ days and it would take only 70 years to accomplish a cycle in which, say, New Year’s Day swept through every month of the calendar! Such a calendar would be completely useless to the farmer. However, we need to ask, Why did they adopt the 360-day year in the first place and why did they not abandon it after the first few years? According to Egyptian records, the 360-day Babylonian calendar was in use for some hundreds of years before the five “epagomenes” or days were added to give a 365-day year. Sir Joseph Norman Lockyer discusses this situation quoting several Egyptian texts and, while he does not say so, the obvious conclusion would be that prior to the Genesis Flood the year was indeed 360 days with 12 months each of 30 days. Whatever caused the Genesis Flood likely also caused the earth to move into a slightly larger orbit about the sun and ruined the perfect calendar given to man in the beginning; time keeping since has then became much more difficult and a permanent reminder of man’s sin.
The Hebrew Calendar. The sacred calendar of the Jews tries to reckon the months according to the moon and the years according to the sun; it is known as lunisolar. The calendar year begins in their month of Nisan [March-April] and has twelve months alternately of 29 and 30 days but this gives a year of only 354 days. However, about every three years [7 times in 19 years] and extra 29-day month, Veadar, is added at the end of their year. That is, between Adar [February] and Nisan. Even so, over a 19-year cycle the Jewish calendar is still 10 days short of the 6939.60 days that the earth takes to orbit the sun in 19 years. For this reason, an extra day is sometimes added to three 29-day months of their year, i.e. Heshvan, Chislev and Adar, to make them 30-day months. All these corrections were formerly done under instruction from the priests located in Jerusalem. Recall that not only must the Passover be kept at the full moon but the planting of crops must always be carried out within the season to ensure a good harvest. Clearly, the lunisolar sacred calendar of the Jews with all the necessary corrections is a most complicated affair. Again, it does cause one to wonder if in a former age it was not the much simpler 360-day year with twelve 30-day months? If all this was not bad enough, the Jewish civil year begins in their month of Tishri corresponding to our September-October.
Early Egyptian Calendars. Again, Sir Joseph Lockyer and many others have shown that from the very earliest times the Egyptians built their pyramids primarily as a means of accurate time keeping. Having an absolutely flat and treeless horizon in a cloudless sky was the perfect environment to observe the moment of the rising sun. The pyramidion on the pyramids and on the pointed obelisk gave sharp shadows on a flat pavement to accurately track the passage of the sun and establish the time of day and the seasons. Even in those early days of history the Egyptian priest/astronomers had discovered that the year consisted of 365¼ days.
Early Roman Calendar. The early Roman calendar was based upon a lunar year of 355 days which is almost exactly 12 lunations that were numbered 1 to 12 in Latin and are still found as the names of our months: September, October, November, and December for the Roman months 7, 8, 9 and 10 respectively. However, to fit this lunar calendar into the year based upon the sun i.e. the solar year, it was necessary to have a cycle of four years of: 355, 377, 355, and 378 days respectively. The year in this calendar began, as did most early calendars, on March 1st to be near the Spring equinox [about March 21st], and this ended at the end of February [Februarius]. However, this early calendar with its years differing in length by over three weeks was useless for farmers to know when to plant their crops.
The Julian Calendar. The Alexandrian mathematician Sosigenes advised Julius Caesar [102-44 B.C.] to introduce a calendar reform based upon the solar year i.e. the time taken for the earth to make one complete circuit around the sun. This reform abandoned any attempt to adapt the years or the months to the lengths of lunations i.e. the moon’s cycles. The new Julian calendar was introduced in 45 B.C and the year began on March 1st. The year was fixed at 365 days while to accommodate the ¼ day, an extra day was added at the end of every fourth calendar year, i.e. the end of February. That year became known as a “leap year” and consisted of 366 days. Even so, because the fraction is slightly less than a ¼ day, the Julian calendar fell behind the equinoxes little by little each year, in fact, it amounted to 1 day every 128 years. In 44 B.C., the second year of the Julian Calendar, the fifth month, Quinctilis, was renamed Julius – our July – in honor of its founder, Julius Caesar. In 8 B.C., the sixth month Sextilius, was renamed Augustus after its successor, Augustus Caesar. The Julian calendar, remained in use until reformed by Pope Gregory XIII in 1582.
Gregorian Calendar. Since Roman times the length of the year was reckoned to be 365¼ days. Greater precision in determination of the length of the year became possible in the 17th century with the invention of the telescope. The procedure consisted of placing the cross-hairs of the eye-piece on a very distant and known star then locking the telescope in that position and waiting an entire year until the same star appeared in the cross-hairs. Pendulum clocks were used at first, atomic clocks are used today. The solar year is now known to be 11 minutes, 14 seconds less than 365¼ days, i.e. 365 days, 5 hours, 48 minutes and 46 seconds or 365.2422 days. However, this precision was unknown to Pope Gregory XIII [1572-1585] but he did know that during the 1257 years from the Council of Nicaea [A.D. 325] to Gregory’s reform  the vernal equinox had drifted back 10 days from March 21st to March 11th.
The length of each day is uniform and was and still is usually measured from sun-rise to the following sunrise; this period corresponds to one revolution of the earth on its own axis while the time taken divided by 24 defines the hour. Following the Babylonian tradition, the circle has always been divided into 360 degrees so that each hour of the earth’s revolution on its own axis is [360 divided by 24] = 15 degrees of longitude. The relative times of daylight and darkness for days throughout the year follow a cycle: Shortest day December 21st; Vernal equinox or day of equal day and night, March 21st; Longest day June 21st; Autumnal equinox of equal day and night, September 21st. The Gregorian calendar year followed the Julian calendar tradition and began on March 1st since this was the month of the Spring or vernal equinox.
By Pope Gregory XIII’s reform in 1582 the Julian calendar had dropped behind the equinoxes by almost 13 days [(45BC to AD1582 =1627 years) divided by 128 = 12.7 days]. Planting crops almost two weeks late could result in disaster for the harvest! Mathematician and physician Luigi Lilio Ghiraldi succeeded by Clavius therefore proposed a variation on the Julian Calendar by adding an extra day every 400 years and this reduced the error to only one day in every 3300 years. The rule of thumb is that if the number of centuries can be divided evenly by four then an extra day is added in the same way days are added in a leap year. Thus, an extra day [Feb. 29] was added in 1600 and again an extra day was added in 2000. The Gregorian calendar was put into operation by Pope Gregory XIII and began October 5, 1582, by moving the calendar forward 10 days to make it October 15th. By this regulation the vernal equinox which then happened to be on March 11th was properly restored to March 21st. Due to religious prejudice against a particularly unpopular pope, the Protestant countries did not immediately introduce these changes. England remained on the old Julian calendar until the reign of William II; it changed in 1752, when the day following September 2nd was decreed to be September 14th. It was at this time that the commencement of the legal year in England was changed from March 1st to January 1st. It will be recalled that the French Revolution had occurred only 3 years earlier  when the beginning of their year was changed from March 1st to September 1st.
The Revolutionaries declared objective was to commemorate the beginning of the French Republic and the end to all Christian celebrations. The Orthodox Church of Russia is still opposed to Gregorian change, still observe the old Julian calendar and, for example, celebrate their Christmas Day on January 6th.
Nit-picking. As previously noted the hour is defined as one twenty-fourth of the time taken for the earth to rotate once on its axis. Time changes from State to State and Country to Country are in increments of one hour or fifteen degrees geographically. Civil time is occasionally adjusted by “leap-second” increments once or twice per year to insure that the difference between a uniform time scale defined by atomic clocks does not differ from earth’s rotational time by more than 0.9 seconds. This is an arbitrary number and was set by international committee. In 1956 in a study of the cesium 133 atom it was found that 9,192,631,770 cycles of radiation occurred during one second; thus entered a new definition of the second. With the introduction of atomic clocks a greater standard of accuracy in timekeeping was introduced, but when tested against the rotation of the earth on its own axis it was found that the earth was slowing down! This is believed to be caused by the braking action of the tides; eclipse data has made it possible to determine the rate of deceleration to be roughly 1-3 milliseconds per day per century. Since 1972 “leap seconds” have been added [none yet subtracted] to the world’s time-keeping clocks to enable civil time to keep in step with the world’s time. While there is evidence that the earth is slowing down no one can be really sure by how much because some of that slow down may be in the cesium clock itself, after all, radioactive decay it is only assumed to be constant.
Encyclopedia Britannica. 1910. New York: The Encyclopedia Britannica Co. Eleventh edition. Volume 4, pages 987-1003 “Calendar”
Encyclopedia Judaica. 1971. New York/Israel: Macmillan Co. Volume 5, Columns 43 to 53 “Calendar”
Lockyer, J. Norman. 1894. The Dawn of Astronomy. London: Cassell & Co. Chapter 24 “The Years of 360 and 365 days”
Richards, E. G. 1998. Mapping Time. The Calendar and its History. Oxford University Press