and the Origins of Astronomy - Nr. 22
Manetho gave the reigns of pharaohs in full years only, leaving the months of partial years unaccounted for. That would be approximately six months per king (the average from 0 to 12), which would extend the total chronology for the first 13 Pharaohs by ca. 6 years. Therefore, I have added 1 year every two kings to the calculated chronology of Manetho.
Figure 15: The First Dynasty of the Old Kingdom of Egypt
The Pharaohs of the Old Kingdom divided the year up into 12 months of 30 days each, plus 5 days at the end = 365 days. Our modern 365-day calendar is similar, except that we add February 29 every four years[52] for a year of 366 days, since the actual year is ca. 365.25 days. Without such a calendric correction, the calendar would quickly come into disarray, lacking one day to actual seasonal time every four years.
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[52] See here e.g. Leap year, Wikipedia.