>>42098664Tiber Septim's ascension to godhood was in the beginning of the Third Era.
Even with his ascension, he can be considered distinct from the other gods of the Nine as he was never part of the act of creating the world. They were alternatively considered the Eight-And-One, describing how Talos differed in his nature from the other Aedra.
Alessia was the first to codify the Eight, back in the days of the first empire, as a way of producing a religion that would be acceptable to the whole empire, by essentially watering down Nord religion, adding some indigenous Cyrodiilic touches, and making sure that Shor/Shezarr wasn't in the main pantheon. (Because he's a very divisive figure in Tamrielic religion; the elves aside from the Dunmer hate him and the humans aside from the Redguards love him.)