I think I'll take this opportunity to delve into the realm of pure hypothesis and mass guessing:
The Miracle of Darna, which occurred in Gran Year 632, would obviously have happened in Naga's lifetime (Seisen kicks off in the year 757, 125 years later). Back in Archanea, Naga died 500 years before the formation of the Holy Kingdom of Archanea. Shadow Dragon et al. starts in the Archanean calendar year 604, 1104 years later. So at the absolute minimum, there is a 954 year gap between the start of Seisen and the start of Shadow Dragon (by which I mean Chapter 1, not the Prologue). Furthermore, consider that Loptous was originally an Earth Dragon of Archanea, and was a participant in the Earth Dragon/Divine Dragon war, which is stated by Archanea's timeline to have happened 740 years before Archanea's foundation (henceforth referred to as BAF for ease); the arrival of Bishop Galle, his blood pact with Loptous and the foundation of the Lopt Cult, events stated by the Jugdral timeline to have taken place in the Gran Year 440, happened in the course of that war. So, if my information and interpretation is correct, this synchronises the timelines at this point: Gran Year 440 for Jugdral, and 740 years BAF for, well, Archanea.
In other words, this places the Miracle of Darna, which happened 164 years later, at 576 BAF; the death of Naga, which happened in 500 BAF, at Gran Year 708; and the beginning of Seisen, ie Gran Year 757, at 443 BAF. This means, assuming that my information and my interpretation and maths is correct, that there is a 1047-year gap between the start of Seisen and the start of Shadow Dragon.
My head hurts.
When compared to the development of the real world in the similarly medieval time period, that actually sounds like a pretty reasonable period of time, though given the presence of magic in the FE universe it's folly to compare these things and hope for accuracy; that presence is very likely to have changed the development direction of the world. It may be impossible to come up with an accurate gauge of the development of technology in the Arcalengdraliris universe over the course of it, given that we generally know nothing about it beyond what we see in game which is basically all war stuff, but I've come to interpret some things as a useful point of comparison: for example, compare the armour of Arden, an armourknight as of the modern Jugdral period, to that of Dolph and Macellan, armourknights of the "modern" Archanea period, to that of... er, whatever Callum is trying to wear. It's not exactly a massive leap in design, but Arden's does seem noticeably simpler than Dolph's and Macellan's, and they're all significantly simpler than Callum's... thing. Of course, from here we still have absolutely no idea how long the timeskip between Monshou and Kakusei is.
So, er, I forgot the point I was making?