Vathanalu Reckoning

The Vathanalu Reckoning is the standard calendar in use throughout most of human civilization in Orithea and Rhydain as well as in certain other parts of the world. "Vathanalu" means "from the disaster" or "from the catastrophe" in classical Tuvai, in reference to the Great Cataclysm, because the calendar counts years starting from that event.
Years of the post-Cataclysm era are notated Cirda Vatha (abbreviated C.V. or CV), meaning "after the catastrophe," and years of the pre-Cataclysm era are notated Batu Vatha (B.V. or BV), meaning “preceding the catastrophe”. The system does have a year zero, the year in which the Cataclysm occurred; it is typically referred to simply as Yalu Vathara, the Year of Catastrophe. The last year before that was 1 BV and the first year after it was 1 CV. The current Vathanalu year is 1255 CV. This corresponds to the year 5106 SAJ by the Tarquenian calendar used in Achuirea.
Like the Tarquenian calendar and almost all the calendars used in Orithea pre-Cataclysm, the Vathanalu Reckoning uses a 364-day lunisolar year divided into 13 lunar months of 28 days each, with the months further divided into four seven-day weeks. Its new year begins on the day after the winter solstice. The sacred portent of the number 13, and the fact that the 28-day lunar cycle fits perfectly into the 364-day solar year to yield 13 months of identical length, are regarded as divinely significant.
The names of the months of the Vathanalu calendar are the same that have been in use by the Vanir-Vaati since before the Cataclysm; in fact, since time immemorial and probably since the days of Ulshanru. They are named for the gods and goddesses of the Thaedonu-Vaati pantheon (plus Touz), with contracted versions of the full names in common usage: 
* Kothaur's Moon or Kothersmoon (first month of the year; begins the day after the winter solstice)
* Touz's Moon or Touzmoon
* Xonoth's Moon or Xonasmoon
* Nevaea's Moon or Nevasmoon (the mid-vernal month; its seventh day is the spring equinox)
* Kaephus's Moon or Kaphusmoon
* Telaon's Moon or Telasmoon
* Auruu's Moon or Aurusmoon (the midsummer month; its 14th day is the summer solstice)
* Tefra's Moon or Tefrasmoon
* Gyrion's Moon or Gyrismoon (sometimes Gyrinsmoon or Gyronsmoon)
* Deinoa's Moon or Deinasmoon (the mid-autumn month; its 21st day is the autumnal equinox)
* Adhanu's Moon or Adhansmoon (sometimes Dhanusmoon)
* Sidra's Moon or Sidrasmoon
* Thrygon's Moon or Thrygasmoon (last month of the year; it's final day is the winter solstice)
Since these are lunar months, each month corresponds perfectly to a full lunar cycle: The new moon always occurs on the first of the month, the full moon always occurs at the midpoint of each month on the 14th day, and the last day of the old moon (the thinnest phase of the waning crescent) always falls at the end of the month on the 28th day.
Religious festivals of the Thaedonu-Vaati, and other festivals and holy days, are scattered throughout the year, but a number of important ones are clustered in two major groups, the midsummer festival season in Aurusmoon and the early-winter festival season in Sidrasmoon and early Thyrgasmoon. A number of important holy days also fall around the onset of spring in Nevasmoon.
Each 28-day month is further divided into four weeks of seven days:
* Odadat
* Dhawadat
* Hirudat
* Tassadat
* Tanodat
* Shamisdat
* Yavarsdat
The common names of the first six are simply archaic Tuvai for 'first day,' 'second day,' 'third day,' etc.; these archaic formulations differ from the modern Tuvai ordinal numbers, but have survived more or less unchanged from ancient times due to being fixed in use in the days' names. The seventh day is named after Yavare of Kalkidan, a semi-mythical lawgiver of the early archaic age whom tradition credits with the decree that the last day of the week be set aside for rest. 
The months and days of the Vathanalu Reckoning calendar predate its annual numbering system, and have been in universal usage across all of "civilized" Rhydain and Orithea since the dawn of human history; however, many different timekeeping schemes for the counting of years were in use before its advent. Some numbered years from the beginning or end of the Schism Wars. Others reckoned the years starting from some supposed date when the early humans left Ulshanru, or even from the creation of the human race; these were completely speculative and varied widely, since historians have no way of knowing how long ago such events really occurred. Some kingdoms used calendar systems based on regnal years or dynastic foundings. Needless to say, this produced a frustrating situation for scholars attempting to make use of books and records from different parts of Orithea. Chronicles from one source might give a year as 7415 since the creation of mankind; another might give the same year as 9294 since the same; a third might call it 3270 since the end of the Schism Wars; a fourth might give it as the 23rd year in the reign of the good king Dagomar; and a fifth as year 449 of the Chalbari Dynasty. 
The near-universal adoption of the Vathanalu reckoning in Orithea and Rhydain was driven by a scholarly consensus (which nevertheless took a number of decades to form after the Cataclysm). This consensus was based on two facts that made the Cataclysm an ideal starting event for a new calendar system. Firstly, its massive destabilizing effects had rippled throughout much of Orithea, not just the lands surrounding the Sarpic Seas; it had an impact on lands as far away as Enyrion and Khadoa. Secondly, its date was clearly and consistently documented by chroniclers from every corner of Orithea, thanks to widely observable phenomena like the darkening of the skies and 'summer without warmth' that followed (even the explosion of Alborak itself was heard and felt, and documented, more than a thousand miles away). Thus a common reference point for the calendar's year zero was available all over the continent.