Equestrian Calendar Concept · 3:38pm Jan 21st, 2015
In writing Act 3, I was re-reading through some of my older notes and discovered a rather disproportionate amount of world-building devoted to a single line in chapter 11.
ECE+121:228(8:23)
Apparently, I had created a relatively complete specification for an Equestrian calendar system, and then promptly forgot about it.
So I'm writing it up here so that a) I don't forget it again, and b) other people can use it if they want.
So, without further ado-
The Equestrian Diarchal Calendar
The calendar design makes a number of fundamental assumptions that may or may be incompatible with other's worldbuilding/headcanons/whatever, so they are listed here-
• Equestria-Earth has a sidereal and orbital period equal to our Earth, or at least acts like it does
• Equestria's moon has phases
• Equestria's moon has a cycle length equal to our own (29.5 days from one full moon to the next)
• Equestria's year has a length equal to our own (365.3 days, aroundabout)
• Ponies can count (should go without saying, but you never know... 
)
The Diarchal calendar consists of two separate calendar cycles, one for the moon, one for the sun.
One lunar calendar cycle (called a 'Month') consists of 29 full days and one half-day. Even-numbered months have the half-day at the start of the month, odd-numbered months have it at the end, so the half-days slot together to make a full day (In the original concept, these double-half-days are a transitional 'changeover' holiday). The lunar cycle mostly exists for ease of scheduling, since it's easier to say and understand 'The fifth day of the eighth month' than 'The two hundred and fifteenth day of this year'
One solar calendar cycle (called a 'Year') consists of 365 days, plus an extra day every 4th year to account for desynchronisation between the calendar and actual planetary motion around the sun (pretty much identical to modern leap years). These days can be further subdivided as convenient. The two I originally used was to subdivide the year into four 91-day groups to correspond with the seasons, and also into 73 5-day weeks (mostly because it subdivides nicely, and also because ponies seem like the kind of easy-going society to adopt a 3-on, 2-off workweek. That, and the 7-day workweek is a mostly human thing centred around historical obsessions with the number 7)
Numbering the calendars is fairly simple, and I shall use the above-mentioned example.
ECE+121:228(8:23)
ECE+ - Denotes the Epoch, 'Equestrian Common Era' in this case. Used to indicate which interesting event you're starting your calendars from (since the solar and lunar calendars aren't synchronised, this is important). For example, Equestria's founding, or the banishment of Nightmare Moon, are two easily usable epoch events.
121:228 - Current solar cycle and solar day. Using the 5-day week mentioned above, it's trivially easy to work out where you are in the week from the date (if the number ends in 4, 5, 9, or 0, it's a weekend)
(8:23) - Indicates the current lunar cycle and lunar day. Lunar cycle counts are loosely tied to the solar cycle, in that you start the cycle count from the start of the solar year (so the count will run from 1 to 13). Since the two calendars are desynchronised, the last month of a given year will almost always roll over into the next year partway through its cycle (so while it may be Day 1, Month 1 of a given year, it may not be Day 1 of that month).
Confusing