Since it's the Demon Slayer World, the development and background are already complete.
Currently on Chapter 5, the 12 Moon Demons was created after Michikatsu older brother of Yoriichi the man who almsit killed Muzan, so we can say that the current timeline, ranges from 150 years ago up to where the series started.
Only problem here is that, the interactions of demons are far from what's supposed to be their nature.
Here Muzan seems like an ordinary boss who cares for his subordinate and demons teaching other demons when it's supposed to be proving that they are higher in terms of hierarchial order.
And it is stated by Tamayo that demons don't tend to form groups, except for Rui Lower Demon 5 as he have Muzan's Permission
Another is the conversation, demons refer to the progenitor of demons as only "Muzan" instead of "Muzan-Sama" or "Lord Muzan". There are also the curse that prevents demons from saying his name so they should only call Muzan as "That Man".
