in Predynastic Egypt - Page 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 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.
 See here e.g. Leap year, Wikipedia.