The earliest Egyptians measured time by the moon; for the month, the interval between two new moons was the unit of time measurement. The moon-month varies in length from 29 to 30, and the lunar calendar is cumbersome and inaccurate. The Egyptians met this difficulty by devising an accurate calendar.
They noticed that the dog star, Sirius, rose heliacally (before sunrise) on July 19th, at the beginning of the inundation (the annual overflow of the Nile). This gave them an improved 365 day calendar.
The 12 months of the Egyptian year were uniformly of 30 days each, giving a total of 360 days, so 5 feast days were added at the end of the year…Each 30-day month was divided into 3 weeks of 10 days each; and there were 3 seasons of the year: the Inundation, the Cultivation, and the Harvest, each consisting of 4 months.
The Egyptian astronomers soon discovered that the true length of the year was 365.5 days, hence the calendar could have been improved by adding an extra day every four years, but this was not done. Ptolemy Euergetes issues a decree in 238 BCE in which is was commanded that every fourth year should be 366 days, but the Egyptians were opposed to this and ignored it. This calendar, established in Kemet over 6,000 years ago, has descended the stream of history to us.