The calamities seem to be fairly consistently paced. 1,500 years, give or take 500 years.

I assume the Void's passage of time is the same as the Source, since it's the closest shard to the Source, to the point that natural fissures between the two open regularly.
Open fissures should synch the time in both worlds, and we know there's been a continuously open one since at LEAST 5,000 years ago, not even looking at all the micro fissure small voidsent get through.

With this assumption, the first, failed attempt was 10,000 years ago.
The 3rd Calamity was about 6,000 years ago.

So eyeballing it, it seems the ballpark of 9,000 years ago for the 1st Calamity, and 7,500 years ago for the 2nd seems about right.