I was digging through the History section of Encyclopedia Eorzea volume 1 (my volume 2 still says it's in Pre-Order?), and it does seem to corroborate that the Umbral Era only seems to last for as long as the calamity itself, with the Astral Era starting as soon as the aftershocks subside and people start rebuilding. The Seventh seems to be a bit of an exception in that the mitigation worked so well, it not only didn't affect the rest of the world much, but people had about 5 years to rebuild before the GC leaders decided it was safe enough to call it. Still enough to trigger a Rejoining, though.