The Aigyptian Calendar, is a fictional calendar featured in the books Eon and Eternity by American SF author Greg Bear.
The calendar is used in Gaïa, a world parallel to ours, where Alexander the Great didn't die young and his empire existed for more than two millennia. The calendar seems to be dominating dating system in Aigyptos (Egypt) and its empire (“Oïkomënë”).
A date notation consists of two numbers: one for the day and one for the year (“140th day of 2345 of Alexandros”). The beginning of the epoch of Alexandros most likely is fixed on either Alexander the Great's birth (356 BCE), or some fictional event of his life (like proclamation of the empire). The last mentioned date is 2349 of Alexandros (about 1993–1994 CE), when the world of Gaïa was destroyed by an alien race known as the Jarts.
To bring the calendar into synchronization with the mean tropical year Hellerick suggested to duplicate every 1507th day (e.g. 140th day would be followed by 140th-bis day). It would produce 1507-year cycle with average year of 365.242203 days.
The new year falls on 13th or 14th of October in the Gregorian calendar, when the Sun's ecliptic longitude is about 200.247°. This longitude is chosen because it takes for the Sun exactly 365.242203 days to reach it again (as for 2000 CE), while for other points of ecliptic (like solstices and equinoxes) the repetition period differs from the mean value. Hence the beginning of the epoch of Alexandros becomes fixed on October 19th, 356 BCE in Julian proleptic calendar.
The table below shows when the recent years of the Aigyptian Calendar begin, and which day is duplicated.
|Year||Begins on||Duplicated day|
|2348||1992-10-13||228th (May 28 and 29)|
|2352||1996-10-13||275th (July 14 and 15)|
|2356||2000-10-13||322nd (Aug. 30 and 31)|
|2361||2005-10-13||4th (Oct. 16 and 17)|
|2365||2009-10-13||51st (Dec. 2 and 3)|
|2369||2013-10-13||98th (Jan. 18 and 19)|
|2373||2017-10-13||145th (March 6 and 7)|
Every four years the duplicated day number is shifted 47 days forward.