There are a number of different calendars operating across the Ewlah Societies. While each of these calendars differ in their starting points, they all share a number of features, the most important of which is that they all make use of a standard orbit (of 380.4 days) as their year, and they all mark the start of the new year on the day of the equinox marking the start of the summer season in the northern hemisphere.
The four calendars referred to in the Kalieda Encyclopaedia are:
The Istran calendar (IC) - this is (allegedly) the oldest extant calendar system, used mainly in the core Istran Lands of Yistrhalhe and Yilstrhe, as well as the islands chains in the Istran Archipilago. The GTC orbit 1200 will be the equivalent of IC 4121. The Istrans claim that the calendar counts orbits from the time of the Starman's second arrival on the planet - though this claim is disputed by archaeologists who have found evidence of human settlements dating back more than 6,000 orbits. Similar to the GT calendar, the Istran calendar incorporates 3 additional leap-days into each eighth orbit, and an additional leap-day every fourtieth orbit, to keep the calendrical count of days in line with the astronomical equinox.
The Falah Standard calendar (FSC) is used by both Vreski and Telik Societies, and originates from a treaty agreed by all nations and empires of that troubled continent some fifteen centuries ago, which set the starting point of the calendar to a supernova explosion in the southern constellations (close to the southern polar axis) four centuries previously. The GTC orbit 1200 will be the equivalent to the FSC orbit 1883. A few of the dates specific to Vreski or Telik events recorded in the Encyclopaedia are given in FSC format, mainly those events which took place more than a thousand orbits ago. The FS calendar differs from the GT calendar in that it adds two leap-days to each fifth year to keep the calendrical and astronomical years aligned with each other.
Most of the dates given in th Encyclopaedia are in the Grand Treaty calendar (GTC) format. The Grand Treaty of Cheidrah brought peace to that continent following more than two centuries of low-level warfare between the major regions. All Bartak, Ambostak and Pentuuk Lands use the GT calendar, as do many other Lands on a more informal basis - because of the Ramajal dominance of the recent communications revolution it has also become an international standard and the de-facto standard of the internet. GTC handles discrepencies between calendrical and astronomical equinox by adding three leap-days to each eighth orbit, and an additional leap-day every fourtieth orbit - GTC 1200 will consist of 384 days, not the standard 380 days. As I write this post, the current orbit is GTC 1199.
The last of the major calendrical systems is the New calendar of the Balhe Lands (NCB). The exact event marking the starting point for the calendar is disputed, though many believe it marks the fall of the Vreski Empire - if such a collapse could ever be determined. Apart from the starting point, the calendar is exactly the same as the Vreski calendar, which is a variant of the FS calendar (the variation is in the division of the orbit into months: an FSC orbit has 16 months, the Vreski and Balhe orbit only 10) - thus two leap-days are added every fifth year. The GTC orbit 1200 will be the equivalent to NCB 430. While the Encyclopaedia uses GTC dating in the text, note that each web page is dated using the NCB system - the date shows the day the page was last updated. The NCB calendar is also used for the Gvekuu web pages - as the game is played professionally in the Balhe Land of Cantane.
Lunar calendars also exist. These are mainly used for religious and ritualistic purposes - for instance the warding systems of the Vreski Empire used to base their calculations of appropriate wards based on the phases and interactions of the two moons.