Hi Franco,
As a speed optimization, we introduced this feature in 1.1.2. Basically, Gossamer Forum "forgets" your new post data for forums that you haven't entered in this many days. For example, if it is set to 90 days, and I haven't entered a particular forum for 90 days, that forum will show up as consisting entirely of new posts.
Calculating new posts is one of the more complex operations in Gossamer Forum. This change was introduced to make it a little easier on the server by not having to keep track of the new posts in forums that users never enter.
Jason Rhinelander
Gossamer Threads jason@gossamer-threads.com