Millennium and Year
The length of the year, and by extension, the millennium should be the same as it is now with almost all calendars, because the length of the year on Earth has been remarkably constant, going almost all the way back to our planet’s beginning.
This makes it a remarkably consistent measure of time, and as such, should constitute the base of this calendar.
However, unlike Asimov’s calendar, the first day of the year is not tied to a specific date on the Gregorian calendar but set at the precise day of the astronomical winter solstice in the Northern Hemisphere. On the Gregorian calendar, this day is currently either December 21 or 22. This makes for a more accurate calendar.
This is modeled after the Solar Hijri calendar used in Iran and Afghanistan, which sets its new year at the precise day of the astronomical spring equinox. This–and not the Gregorian calendar, you Eurocentrists–is the most accurate calendar currently in use in the world. Both calendars and this one have 365 day regular years and leap years with 366 days. The Gregorian calendar sets its leap years at regular four year intervals, with exceptions made for years ending in 00 where the century is not divisible by four (i.e. 1700, 1800, 1900, 2100). However, by simply tying the first day of the year to the actual day of the winter solstice, the calendar never varies more than about a half day from the actual completion of one Earth orbit around the sun.
This, however, means that the incidences of the leap year are slightly more irregular than the Gregorian Calendar. Most of the time four years pass between leap years, but occasionally the interval is five years. In the Earth Epic calendar, the last leap year was 11.717 (starting on 12/21/2016) and the next one will be on 11.721 (starting on 12/21/2020). The last five year interval between leap years was between 11.708 and 11.713 and the next one will be between 11.741 and 11.746.
The Millenium itself is one of three units in the Earth Epic Calendar that deviates from the “rule of one hundreds” in the Earth Epic Calendar, because a millenium equals 1,000 years. The other units that diverge are the Epoch, and Quarter.
—> next section: Quarter