the Selian calendar is quite different from Earth calendars because the planet is rather different. Suenu is an unremarkable terrestrial planet with a nitrogen-oxygen atmosphere, geological activity, and vegetation covering two of its three continents. Differences to earth include the fact that the day lasts 1296 earth hours, giving you something like a month-long day and a month-long night, the year lasts 5171 hours, making it so that each astronomical year lasts just under 4 day-night cycles. Every year, the first day comes a bit over 13 hours later, which means that for the equinox to occur at the same time of the day it takes around 397 days [the 27 earth days planetary revolution that is the day of Suenu, not the 24 hour earth day]. Seasons are also unheard of in the planet because it's eccentricity is really low and because the planet's equator has no discernable tilt.
One of the biggest challenges for the inhabitants of Suenu, therefore, is to measure small units of time. Since the only season-like phenomenon is the day-night cycle, that comes with extreme reliability, there's no great need to keep track of seasons and planetary revolutions, although it's still of great interests to astronomers, navigators, traders, and timekeepers [a highly relevant job, mind you]. small units of time are, obnoxiously, measured differently by different cultures, and often even local towns. Since there's no obvious astronomical way to measure any relevant unit of time, each town or city uses its own clocks: during the day it's quite easy to do so: you just need to track the movement of the sun and follow it across it's path to measure time in the order of 10 or 20 hours, shadow clocks or a dark room with a single opening for the sun to come in work fine for that. During the day, you can do the same with a star, but since the day and the year don't neatly line up, you really need to know your calendar to keep time at night.
The Selian calendar, the one used by the focal culture of the conworld, works in this manner.
The first astronomical unit is the day, lasting, as has been said,
[windows crash, this post is draft]