For this in its most complete and final form, the world is indebted to 'Julius Caesar, who, during his office as Pontifex Maximus, undertook the memorable task known as the "reform of the calendar." The Roman year had hitherto consisted of 355 days, with a month of thirty days intercalated every third year, so that the average, length of the year was 365 days. This arrangement was attributed to Numa Pompilius, who added two months to the short year of Romulus; its regulation was left to the pontiffs. If the intercalation had been regularly made, the Romans would have lost nearly one day in every four years, since the real length of the solar year is about 365 days; but the business was so carelessly executed that the difference between the civil and the solar year sometimes amounted to several months. Caesar called on the astronomers, especially on Sosigenes of Alexandria, to rectify the discrepancy and prevent future error. It was determined to make the first of January of the Roman year U.C. 709 coincide with the first of January of the solar year which we call B.C. 45. But it was calculated that this Jan. 1 of the year U.C. 709 would be 67 days in advance of the true time; in other words, it would not concur with Jan. 1 of B.C. 45, but with Oct. 22 of B.C. 46. Two intercalary months, making together 67 days, were therefore inserted between the last day of November and the first of December of the year U.C. 708. An intercalary month of 23 days had already been added to February of that year, according to the old method. The Roman year 708 was thus made to consist in all of the prodigious number of 445 days (i.e. 355+-2 +67). It was hence scoffingly called "the year of confusion ;" more justly it should be named, as Macrobius observes, "the last year of confusion." To prevent future errors, the year was lengthened from 355 to 365 days, each month except February being lengthened (by one or two days, nearly alternately), according to the rule which we still observe. But as the solar year consists of very nearly 365+ days, it was manifestly necessary to add one day in every four years, and this was done at the end of February, as at present in our "leap year." Such was the famous Julian Calendar, which, with a slight alteration, continues in use in all Christian countries to the present day.
Gregorian Calendar. — The addition of one day for every four years would be correct if the solar year consisted exactly of 365+ days, or 365 days and 6 hours; but, in fact, it consists of only 365 days, 5 hours, 47 minutes, 51½ seconds; so that the Julian year is longer than the true solar year by about 12 minutes. Caesar's astronomers are supposed to have been aware of this, but to have neglected it. Accordingly, in the year A.D. 1582, the beginning of the Julian year was found to be about 10 days behind the true time, the vernal equinox falling on the 11th instead of the 21st of March, its date at the Council of Nice, A.D. 325. The time of Easter, therefore, and of the other movable festivals, had been unsettled by the progressive recession of days, and it was matter of importance for ecclesiastical as well as civil purposes that the calendar should be rectified. Pope Gregory XIII (q.v.) therefore "ordained that ten days should be deducted from the year 1582, by calling what, according to the old calendar, would have been reckoned the 5th of October, the 15th of October, 1582; and in order that this displacement might not recur, it was further ordained that every hundredth year (1800, 1900, 2100, etc.) should not be counted a leap-year, except every fourth hundredth, beginning with 1600. In this way the difference between the civil and the natural year will not amount to a day in 5000 years. In Spain, Portugal, and part of Italy the pope was exactly obeyed. In France the change took place in the same year, by calling the 10th the 20th of December. In the Low Countries, the change was from the 15th of December to the 25th; but it was resisted by the Protestant part of the community till the year 1700. The Romanist nations in general adopted the style ordained by their sovereign pontiff; but the Protestants were then too much inflamed against Romanism in all its relations to receive even a purely scientific improvement from such hands. The Lutherans of Germany, Switzerland. and, as already mentioned, of the Low Countries, at length gave way in 1700, when it had become necessary to omit eleven instead of ten days. A bill to this effect had been brought before the Parliament of England in 1585, but does not appear to have gone beyond a second reading in the House of Lords. It was not till 1751, and after great inconvenience had been experienced for nearly two centuries, from the difference of the reckoning, that an act was passed (24 Geo. II, 1751) for equalizing the style in Great Britain and Ireland with that used in other countries of Europe. It was then enacted that eleven days should be omitted after the 2d of September, 1752, so that the ensuing day should be the 14th." Russia still adheres to the Old Style, so that her reckoning is now 12 days behind that of the rest of Europe.
Calendar of the French Republic. — By a decree of the National Convention, on November 24, 1793, it was ordained that a new era should date from the beginning of the Republic, September 22, 1792; the midnight preceding which, being the autumnal equinox, was fixed upon as the epoch, from which the years were to be reckoned as Year One, Year Two, etc. The year was divided into 12 months, each of 30 days, to which new names were given, as Vendemiaire (vintage month), Brumaire (foggy month), Frimaire (frost month), etc. The months were divided into periods of 10 days, called Primidi, Duodi, Tridi, etc. The tenth day was to be the day of rest, the Christian Sabbath being done away. Five intercalary days were added for each year, viz. the festivals 'of Genius, Labor, Action, Reward, Opinion. In every fifth year there was to be an intercalary festival of The Revolution. This calendar went into use Nov. 26, 1793, and was abolished in 1805 by Napoleon, who ordered the Gregorian Calendar to be resumed on Jan. 1, 1806. — Carlyle, French Revolution, 2:336; Penny Cyclopcedia, s.v.; Chambers's Encyclopcedia, s.v. SEE CHRONOLOGY.