The best source I can think of is the Glorantha, Introduction to the Hero Wars book from Issaries - still available as a PDF. It has a whole chapter on Malkionism. While it is a third age supplement, it has a detailed history with dates so it is not that hard to get an idea of what parts apply to a given time period.
I don't think the year the Mongoose Second Age material is set in has been officially anounced yet, though I think around 980 sounds like something that has been thrown out there.