Relative Time was established by the University League when it was first founded, and League historians and temporal engineers collaborated to create a timeline of human history that would make sense of the far-flung human adventure.
Terrestrial history was all but irrelevant. The Hub Civilization that emerged in the wake of the Colonial Wars is certainly a descendent of the human cultures that originated on Terra Prima, but the long detour via the Altair and Procyon colonies broke the continuity of human relationships with Terran cultures. The relationships between the various human ethnocultural groups that formed the matrix of the Hub, and their Terran forbears, are of interest only to dim historical and anthropological scholars toiling on Markadam and Allskander.
Relative Time is naturally complicated by the vagaries of planetary experience. Even in the (comparatively) closely-clustered worlds that make up the Hub, interstellar distances and the imperfect technologies that bridge them inject great ambiguity to temporal concepts. The highly-tuned mathematical models and navigation calculators that enable interstellar ships to use beacon technology to arrive more or less when they should at a given destination aren’t well-suited to describing the march of history.
Still, the historians had to have a reference point. They chose the founding of the first known viable human colony: Altair III.
It must be remembered that at the dawn of its Interstellar Migration period, Terra Prima had mastered only the most primitive form of transit, a proto-subspace drive that enabled supralight travel and allowed only the most elementary three-dimensional navigation. In addition, practically nothing was known about the shape and fabric of space itself. Our Terran ancestors hypothesized the temporal distortion of interstellar distances on a linear model. But they knew that it would be a one-way trip for the initial colony ships. Over a dozen were sent out, that we know of—possibly many more. Records from the era are fragmentary at best, even on Terra Prima itself.
Relative Time is used mainly for scholarly and historical recording purposes. For common reckoning, a number of systems predominate in the Hub, with the most widely-used being the SD (Standard Dating) system. As temporal distortion makes date reconciliation among member worlds challenging (to say the least,) most dates are appended with an additional prefix to identify major colony nodes that share a generally accepted dating structure.
For navigational purposes, where greater precision is required both relative to a reference point and to elapsed time, the standard hour is universally used. This usage has spread to many common applications, particularly in the transport sector, where all transit times are estimated in hours.
Most colonies also have a local referent system of dating, usually pegged to their Charter date.