The Thirteenth's time line flow (POSSIBLE SPOILERS)

[MrThinker]

Definitely stated in the lead-up promotion to Shadowbringers, and probably within the story as well.

Also I am of the understanding that the first calamity was the Sundering, or at least the aftermath of it.

JP Fanfest, when we found out we were heading to the First, Yoshi P gave a brief history of the Sundering and the Calamites, in which he stated that the Sundering was 12000 years before the start of ARR. Unfortunately, the timeframe is not mentioned in game and we only have this word of god info.

Also, no the Sundering is NOT the first calamity. The first thing that happened post Sundering was the fall of the Thirteenth and it's transformation into the Void, then the Ascians figured out the aetherial connection between Shard and Source that they used in each subsequent rejoining.

There is a period of time between Sundering and First Calamity that we know nothing about on the Source. "The Age of Gods", originally used to refer to anything prior the First Calamity, should now apply to this era of no known records, where the recently sundered had to get used to their new forms and vague memories.