The Unified Human Calendar is very much not what it claims to be. Humans are a distinctly disunited people, and timekeeping is no exception. It also wasn't invented by humans, nor do humans often consider important- rather, it's a xenocultural studies tool. Neither is it actually a calendar- though it uses a unique one. The Unified Human Calendar (UHC) is, most properly, a timeline of human events plus the works they are sourced from and the methodology used to determine the year of the event. An example is that of the year labeled 49178 (in the gulf before the current bloom- evidence suggests human spacefaring occured in at least 2 prior blooms, but the UHC is dated from its earliest event with a definite year), in the excerpt which follows:
"49178: On the homeworld Uelandi (a.7000-A), the food-preparator Shebele discovers a way of cooking the Vatte fruit to produce something edible rather than medicinal, which reverses several ongoing famines (49178-1); On the homeworld Artay (a.18000-A), the priestess-poet En-hedouan writes her Nymshara (49178-2);...
a.7000-A: Uelandi is one of several human homeworlds that is mostly inaccessible from the Ecumene, but is close enough that it is possible to take a pilgrimage there- and many of those that call Uelandi their homeworld do. Uelandi lies approximately four kilolightyears hubward-antispinward of the Ecumene, and is only 210 lightyears from Dhana/Aridat/Kashimue.
a.18000-A: The name of Artay is a best-guess- apellations for the planet vary greatly, but are often related to words for land in the language they come from. Artay- or at least the Prime Artay- is considered likely to exist approximately ten kilolightyears antispinward of the Ecumene and orbit a Y3V star called Seul, but beyond that information is largely trivia."
The UHC uses its own derivative of the Union Day for its calendar- a simple version with 10 fewer days and no year subidivisions, considered by the editors of the UHC to match the human homeworld's orbital period.