Sosigenes of Alexandria was the astronomer consulted by Julius Caesar for amending the Roman Calendar, which was around 46 BC 67 days in advance of the true year.

So what do we know about Sosigenes today? He was doubtless an Alexandrian Greek. It is possible that Sosigenes was in the entourage of Cleopatra and since Caesar was himself lauded in history as an amateur of both beautiful women and astronomy, he was quite probably inclined to consult Sosigenes. Using his authority as ruler of the Roman world he accomplished the reform. The calendar was probably designed by Aristarchus about 200 years earlier on the basis of an calendar which had been introduced in Egypt by the Ptolemaic dynasty (a Hellenistic royal family which ruled over Egypt for nearly 300 years, from 305 BC to 30 BC) nearly two centuries before. Sosigenes just adapted it for the mighty Romans.

The year 46 B.C., Julius made to consist of 445 days: it was called the year of confusion; but it was more properly the last year of confusion. The reformed year began, not on the 25th of March, but 1st January, 45 B.C. The new year was one of 365 days, with an additional day for every fourth year, in February. The alternate months of the year (January, March, May, July, September, November), were to consist of 31 days: the intervening months were each to be of 30 days (February being 29, except in leap years). This symmetrical arrangement was upset by the vanity of Augustus in 27 B.C., when he gave his own name to the 8th month, then added the day he took from the 9th, and otherwise varied the lengths of the months into their present irregularity.

The Julian year of 365¼ days was too long by 11 minutes 12 seconds. This must have been known to Caesar and to Sosigenes; as more than 100 years before, it had been proved by Hipparchus, whose calculation was within 4 minutes of the truth. Hipparchus had calculated that the error would amount to a day in 300 years; but it seems that the error is more than double and would amount to a day in 128 years. Caesar and Sosigenes doubtless considered that the error might be left to the future to correct. They could hardly anticipate that it would be binding on Western Europe for 16 centuries, and on Eastern Europe for nearly 20 centuries.

The Julian Calendar, as deformed by Augustus, governed Christendom until 1582, when Pope Gregory XIII, by the advice of Lilio and other astronomers, struck out the ten days then in excess, and reformed the Calendar of Julius by an order, that the last year of each century should be a leap year only when it is exactly divisible by 400. That is to say, 3 leap-years are suppressed in every 4 centuries. This is today called the Gregorian Calendar.

At a synod of Eastern Orthodox churches in Istanbul (Contantinoupolis) in May 1923, the Serbian scientist Milutin Milanković, an astronomical delegate to the synod representing the Kingdom of Serbs, Croats and Slovenes, proposed the Revised Julian Calendar for adoption by the Orthodox churches , and the rest is current affairs to this day.

The synod synchronized the new calendar with the Gregorian calendar by specifying that the next October 1 of the Julian calendar would be 14 October in the Revised Julian calendar, thus dropping thirteen days. It then adopted a Leap year rule that differs from that of the Gregorian calendar: Years evenly divisible by four are leap years, except that years evenly divisible by 100 are not leap years, unless they leave a remainder of 200 or 600 when divided by 900, then they are leap years. This means that the two calendars will first differ in 2800, which will be a leap year in the Gregorian calendar, but a Common year in the Revised Julian calendar.