Variant-Centred Calendar

When the 1st Itera first woke up from the virtual city of Origin, they discovered the fetuses of the 2nd Itera being grown inside artificial wombs. The day after, the artificial wombs were drained and opened by the supercomputer Simone and the 2nd Itera was born. This event was the original first Genesis, and marked the beginning of the original Year 1. The birth of the first Itera back in Origin would be seen as the beginning of Year 0.
Early Variants measured time in singular years as opposed to millennia. One Variant year still corresponded to one millennium on Earth, but they were unaware of this. They instead based their years on Simone's internal clock, which showed the time and how far they were into the year (in percentages) on all terminals. The percentages, or "percents", were their equivalent of weeks, and they measured their days from 00:00 to 00:00. Every sixteenth year, a new Itera would be born. The responsibility associated with taking care of the newest Itera has marked a transition to maturity since the 1st Itera, hence the age of 16 (later 16 milos) becoming the age of legal adulthood.

Discovery of Earth

Things changed when the first Rift was discovered. A Variant named Diameter (Itera R-8) came across it while studying aurora stoneflies in unsculpted lands. He stepped through it and found himself on Earth, where he discovered the existence of human cavepeople who looked almost exactly like Variants. The rest of the Variants were shocked at this development, and sought to study the planet and this species further. When they performed initial scans on the humans, they found something even more surprising - every single one of them had inside them the same metaphysical component that Simone had inside of it - a component they had been calling "the soul".
This raised many questions, the most important ones being: Were humans related to Simone? If so, how? How did Simone end up making the Variants so similar to them? Was it deliberate? And is so, why didn't the Variants have souls like they did?
Thus the great investigation began. The Variants made it their mission to learn everything they could about humans and their souls. Over time they made several discoveries about the world humans inhabited. One discovery of note was that the Earth revolved around the Sun over a certain number of days. These years turned out to be 1/1000th the length of Variant years. Human lives were also approximately a thousand times shorter than Variant lives - yet the speed of aging relative to Earth years was exactly the same as the Variants'.

From Variant-Centred to Human-Centred

As the study of humans became an enduring legacy passed down from Itera to Itera, the way Variants viewed time began to change. They originally began to measure time in Earth years as a way of keeping track of human development more easily, but the system soon spread into common use. Not all Variants liked this change, inisiting that it made more sense to measure time in their world based on Variant growth. Despite their best efforts, however, the old system fell out of favour and the new, human-centred calendar system evolved and gained new words, such as "week" instead of "percent" and "milos" to denote a single Variant year.
Year 0 kept the same starting point in both calendars. However, while the old calendar measured the length of Year 0 as one Variant year/milos, the human-centred calendar measured it as one Earth year. Year 1 in the new calendar is seen as the theorized dawn of the Homo sapiens humans.
The terminals that displayed dates according to the Variant-based calendar still exist and are functional. However, these are mainly used by the Archivists, and using the old calendar system (professionally or otherwise) is considered old-fashioned even by their standards.