Aigyptian Calendar

The Aigyptian Calendar is a fictional calendar inspired by the books Eon and Eternity by American SF author Greg Bear. The book Eternity deals with an alternate universe in which Alexander the Great didn't die young and his empire didn't fragment upon his death.

To allign with the solar year, every 1507th day (4 years, 47 days) is repeated. It would produce 1507-year cycle with average year of 365 + ³⁶⁵⁄₁₅₀₇ (365.242203) days.

Calendar Wiki user Hellerick made the Calendar Wiki article and the calendar's repeated day rule. This program calculates when the repeated days will occur in comparison to the Gregorian Calendar.

Development Log

• 12/11/20 - Date input code completed
• 12/11/20 - Calendar context information added
• 12/29/20 - Past and Future date calculation code completed

Organization of the Calendar

Here are several ways the calendar can be organized, with the 11th day being repeated, for example.

Days Repeated

Year 0 the Aigyptian Calendar is 356 BC, (Birth of Alexander the Great). Based on the locations of leap days, the first leap day on the calendar (using 356 BC as the starting year) is:

• Sat, 17 Sep 356 BC Gregorian
• Fri, 16 Sep 356 BC Aigyptian Date
• Since 356 BC is a leap year according to the Gregorian Calendar, the repeated day is shown as one day behind the Gregorian calendar, as the leap day has to be undone from Feburary 29, and redone at its calculated location.

20/21 July 356 BC – 10/11 June 323 BC

Differences in Calendar Day

• If the repeated Gregorian day is in a common year (which it usually is), the Aigyptian Calendar and the Gregorian calendar read the same day. This is because the year the repeated day occured occurs after a Gregorian leap year. This is reflected by not including that date in the second column seen on the page of the list of dates, accessible by clicking the orange "Past Dates" button or the blue "Future Dates" button. This distinguishes the Aigyptian leap days not during Gregorian leap years.
• If the repeated Gregorian day is in a leap year and is after Feburary 29, the repeated Aigyptian day is one calendar day before the Gregorian date, as there the leap day on Feburary 29 must be undone, as the Aigyptian calendar has leap days in different locations relative to the calendar each year. After the repeated day on the Aigyptian Calendar, the Gregorian and Aigyptian calendar are on the same day.

Here's an example of this process, with the Gregorian Calendar on top and the Aigyptian Calendar on the bottom with March 3 as the repeated day. The Februrary leap day must be undone in the Aigyptian calendar and the redone onto March 3 to allign with the seasons.

Between 356 BC and 275757 AD (276113 years):

• The Gregorian Calendar will be behind ²⁷⁶¹¹³⁄_{3030.30303012} = 91.1172900055 days
• The Aigyptian Calendar will be behind ²⁷⁶¹¹³⁄_{30303.0302751} = 9.1117290084 days