Late in the late post-collapse period- that is, the early 3000s- a peculiar star was discovered in sector 423. Hidden behind a lane or two of dust, one of the few stars capable of going supernova ever to be discovered within the Ecumene shone. This already made it an interesting star, but what made it rise to real prominence was the planet that orbited eighty-three astronomical units from the star- a heavy but fully-formed garden world. Evidently it had been captured by the giant a scant few million years ago- a mercifully short blip of extinction in the palaeontological record stood between two periods of warmth- but now it was habitable, if mostly barren. Free oxygen circulated in the atmosphere, though the life that produced it had been displaced by hardier distant relatives.
An especially barren continent far from anything else was an obvious candidate for inhabitation- and as scientists and thrillseekers flooded into the system, cities sprung up. As this was an era where ironic and absurd names were in fashion, the star was dubbed Yclathena (a poetic word for a red dwarf, literally frugal star in a language local to sector 423) and its planet named after the Zhadny fruit that grows on its surface (scientifically calculated to be the least spherical fruit on the planet). With the colony came a calendar system, and in the same fashion it was made to be distinctly useless to those who grew up off-planet.
Zhadny orbits Yclathena in almost exactly 186 Concord years, leading to the expectation of yet another Concord-multiple calendar... but instead it is divided into nine periods of unequal length. Three are thirty-one years long, and each is preceded by an eighteen-year period and followed by a thirteen-year one. Due to a near-alignment of orbital and axial summer on the main continent, this works out to- a long summer and spring, a short winter, and a short bifurcated autumn. Of course, it wouldn't do to have a normal year length- instead, a figure of 240 days is used; meaning that the 31-year period is actually 46 and a half short years, the 18-year period is actually 27 short years, and the 13-year period is actually 19 and a half short years. Each of these short years is divided into 16 periods of 15 days- it originally alternated between that and 15 periods of 16 days, but simplified over the years because the planetary day is close to 5 standard days. It has been approximately 30 long years since settlement, and even now the people of Zhadny follow their ridiculous calendar.