Altear is an alternate earth. There is no branching off point, it's simply a fantastical version of our world. Altear is somewhat steampunk, with some modern-day technologies, others fitting more with the early and mid 20th century, and others dating from before then.
The timeline of this world is split into eras of varying lengths, changing "eras" whenever there is a significant development. If an event is significant enough, it may change the name of the previous era. For example - PS stands for Pre-Shift, but was formerly known as AT - After Tyrant, referencing the fall of the tyrant Willahelm. Because of this, 343 AT and 1 PS are the same year, as the Shift happened 343 years after the fall of Willahelm.
The Shift references the Catago taking over, seizing control of the majority of the world's governments and ruling with an iron fist.
Altear is a single, Pangea-like continent with no countries, instead having independently ruled cities, towns, and villages with large rural areas stretching between them.