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The previous Roman calendar consisted of 12 months with a total of 355 days. In addition, an intercalary month Intercalaris was inserted between February and March, usually in every second or third year. Intercalaris was formed by adding 22 days to the last five days of February, creating a 27-day month. Since it began after a truncated February having 23 or 24 days, it had the effect of adding only 22 or 23 days, forming an intercalary year of 377 or 378 days.
If managed correctly, this system allowed the Roman year, on average, to stay roughly aligned to a tropical year. However, at times the calendar was allowed to drift badly, and special steps had to be taken to realign it. One such period was during and after the Second Punic War. This happened again during Julius Caesar's pontificate, 63 BCCenturies: 2nd century BC 1st century BC 1st century Decades: 110s BC 100s BC 90s BC 80s BC 70s BC 60s BC 50s BC 40s BC 30s BC 20s BC 10s BC Years: 68 BC 67 BC 66 BC 65 BC 64 BC 63 BC 62 BC 61 BC 60 BC 59 BC 58 BC Events Roman conquest of Palestine; the p to 46 BC, when there were only five intercalary months, whereas there should have been eight, and none at all between 52 BCCenturies: 2nd century BC 1st century BC 1st century Decades: 100s BC 90s BC 80s BC 70s BC 60s BC 50s BC 40s BC 30s BC 20s BC 10s BC 0s BC Years: 57 BC 56 BC 55 BC 54 BC 53 BC 52 BC 51 BC 50 BC 49 BC 48 BC 47 BC Events Milo is tried for the murder of Clod and 46 BC. Thus by 46 BC a calendar date was occurring three months earlier than it should have against the tropical year.
The Julian reform was intended to correct this problem permanently. Before it took effect, the deficit was corrected by inserting 67 days (22+23+22) between November and December of 46 BC in the form of two months, which are now sometimes referred to as Unodecember and Duodecember , in addition to 23 days which had already been added to February. Thus 90 days were added to this last year of the Roman Republican calendar, giving it 445 days. Because it was the last of a series of irregular years, this extra-long year was, and is, referred to as the last Year of Confusion. The first year of operation of the new calendar was 45 BC.
Despite the new calendar being much simpler than the Roman calendar, those tasked with implementing it, the Pontifices, apparently misunderstood the algorithm. They added a leap day every three years, instead of every four years. This resulted in too many leap days. AugustusAugustus (plural Augusti is Latin for "majestic" or "venerable". Although the use of the cognomen "Augustus" as part of one's name is generally understood to identify the Roman Emperor, this is somewhat misleading; "Augustus" was the most significant name Caesar remedied this discrepancy by skipping several leap days after 36 years of such mistakes.
The historic sequence of leap years (i.e. years with a leap day) in this period is not given explicitly by any ancient source, although the existence of the triennial leap year cycle is confirmed by an inscription that dates from 9 or 8 BC. The chronologist Joseph Scaliger established in 1583 that the Augustan reform was instituted in 8 BC, and inferred that the sequence of leap years was 42 BC, 39 BC, 36 BC, 33 BC, 30 BC, 27 BC, 24 BC, 21 BC, 18 BC, 15 BC, 12 BC, 9 BC, AD 8, AD 12 etc. This proposal is still the most widely accepted solution. It has also sometimes been suggested that 45 BC was a leap year.
Other solutions have been proposed from time to time. Kepler proposed in 1614 that the correct sequence of leap years was 43 BC, 40 BC, 37 BC, 34 BC, 31 BC, 28 BC, 25 BC, 22 BC, 19 BC, 16 BC, 13 BC, 10 BC, AD 8, AD 12 etc. In 1883 the German chronologist Matzat proposed 44 BC, 41 BC, 38 BC, 35 BC, 32 BC, 29 BC, 26 BC, 23 BC, 20 BC, 17 BC, 14 BC, 11 BC, AD 4, AD 8, AD 12 etc., based on a passage in Dio Cassius that mentions a leap day in 41 BC that was said to be contrary to (Caesar's) rule. In the 1960s Radke argued the reform was actually instituted when Augustus became pontifex maximus in 12 BC, suggesting the sequence 45 BC, 42 BC, 39 BC, 36 BC, 33 BC, 30 BC, 27 BC, 24 BC, 21 BC, 18 BC, 15 BC, 12 BC, AD 4, AD 8, AD 12 etc.
In 1999, an Egyptian papyrus was published which gives an ephemeris table for 24 BC with both Roman and Egyptian dates. From this it can be shown that the most likely sequence was in fact 44 BC, 41 BC, 38 BC, 35 BC, 32 BC, 29 BC, 26 BC, 23 BC, 20 BC, 17 BC, 14 BC, 11 BC, 8 BC, AD 4, AD 8, AD12 etc., very close to that proposed by Matzat. This sequence shows that one aim of the Augustan reform was to ensure that key dates of his career, notably the fall of Alexandria on 1 August 30 B.C., were unaffected by his correction.