As far as I recall, the black dust appearance began shortly after the Winter Solstice, but was only noticed about a sevenday before the first threadfall of this Pass and after Ramoth's egg clutched. When the weather turned warmer, there were a few threadfalls, and then Lessa went back in time. So I'm thinking this is more or less where we are in the timeline.
In any case, the boys have missed the Impression of Ramoth's first clutch, but that means they'll also not be subjected to the back-in-time expedition to the Southern Continent, which had some negative consequences initially, but also caused a lot of great events in the future and the ultimate destruction of the threat of Thread.