Ancient egyptian dating system
This civil calendar was split into twelve months of 30 days with an additional five epagomenal days attached at the end of the year.These additional five days were considered to be unlucky.
(inundation) occurred at the heliacal rising of a star they called Serpet (Sirius).
It has been calculated that this sidereal year was only 12 minutes longer than the mean tropical year which influenced the flooding, and this produced a difference of only 25 days over the whole of Ancient Egypt's recorded history.
The Egyptians noted that the heliacal rising of certain stars, such as Sirius and Orion, matched the first day of the 36 successive decades and called these stars decans.
During any one night, a sequence of 12 decans would be seen to rise and was used to count the hours.
(This is most unusual since other civilizations of that era are known to have started months with the first setting of the new crescent!