The Egyptian Civil Calendar is an ancient solar dating system that was used for civil purposes. The calendar had twelve months of thirty days each. Each month had three divisions or weeks of ten days. At the end of the year, an additional five epagomenal days, each named after Egyptian dieties, completed the annual cycle.
The rising of Syrus actually occurs at an interval of almost exactly 365.25 days. Therefore, every four years, the event would occur one calendar day later. Every 1461 Egyptian years, Syrus would once again rise on the same calendar day. Also, since there are actually 365.24224 days in a true tropical year, the New Year's Day of the ancient 365-day civil calendar drifted through the seasons. For this reason, it is referred to as a wandering calendar. In 1507 tropical years, the Egyptian calendar would have cycled 1508 times.
As mentioned, every four years, the Egyptian New Year, Thoth 1, occurred one day earlier than the rising of Syrus. In 1,461 Egyptian years, the Calendar would have cycled 1,461 times returning Thoth 1 to the initial point in the cycle again. This is known as the Sothic Cycle. Many consider that the Sothic Cycle starts and ends when Thoth 1 coincides with the helical rising of Sirius at an established viewing position in Egypt. The helical rising of Sirius was closely associated with the yearly inundation of the Nile River.
Even though this calendar wandered through the seasons, it was a simple calendar, and recorded dates are easily transferable to Julian or Gregorian dates. The Egyptians seemed to prefer constancy in their civil calendar rather than the irregularities of intercalation and leap years. This is important for our study of Ptolemy's Canon of Kings.
Claudius Ptolemy used the Egyptian Calendar when dating the reigns of kings. Ptolemy's Cannon of Kings listed kings dating as far back as the first year of the Babylonian ruler, Nabonassar, which was Feb 26 (Thoth 1 by Egyptian reckoning.), 747 BC.
Egyptians reckoned days from sunrise to sunrise and used the non-accession-year method for dating kings. When a king died and a new king ascended to the throne, that part of the calendar year during which the old king was still alive was considered to be the last year of his reign, and the remainder of that calendar year was considered to be the first year of the new king. Other calendars, such as the Babylonian, Persian and Hebrew dating systems, used the accession-year method for dating purposes and would have considered that year to have been the accession year of the new king. His first year would not have begun until after the following new calendar year.
The non-accession-year method didn't fit as neatly on a number line as the accession-year method, but when one recognized that such a dating system was used, then reckoning wasn't really so difficult. Throughout history, Judah generally used accession years, while Israel sometimes used the accession year system (postdating) and sometimes used non accesion (antedating) reckoning of time.
The Egyptian Civil Calendar simply repeated every 365 days without any intercalation. Censorinus wrote that an Egyptian New Year fell on July 20, 139 AD according to the Julian Calendar handed down to us. Therefore, when historical manuscripts, relics and artifacts using Egyptian dates are discovered, the equivalent Julian date can easily be determined.