1371-1372 comes from various sources in the game. I believe even the records you find on your way down into the Underdark give you an idea of when it all happened.
So Moonhaven was raised, it seems, when the Dark Justiciars raided it back in Uktar 1372.
It's also according to some documents found in the grove.
But honestly, I've had my doubts that Larian has had two groups working on this, neither realizing what the other is doing because some elements seem like Moonhaven was destroyed recently while others imply 120 years ago.
It's like Aradin and Halsin and their expedition. For the longest time I thought they'd only been gone maybe a week and the tieflings we're maybe in the Grove for a few weeks tops. Looking at the timeline in the different records (some you have to steal) you find out Halsin's been gone like a month and a half and the tieflings arrived two month ago.
But the temple is five minutes jog away! What the heck has Halsin and Aradin been doing for over a month and a half, and how were they NOT discovered until just before you show up at the Grove? A month and a half? Really? I had a hard time reconciling that one, creating for myself my own set of catacombs deep under the temple and somehow Aradin and his crew managed to slip in past the goblins and got lost in there for a month and a half, trapped in the nebulous catacombs I created in my mind hoping to find some secret path into the Underdark.
Honestly, it doesn't at all make sense. Even IF you pretend the temple is miles away, it shouldn't take them a month and a half.
So, I fear the same thing has happened with Moonhaven. Some group made it thinking it was destroyed recently and another that wrote the tomes and books thought it was destroyed during the Time of Troubles.
