No, it won't delete anything - it is an optimization that was added quite some time ago to forget the "new" data (i.e. how many "new" posts are in a forum) after a certain number of days. That is, if I enter the "Chit Chat" forum once, and don't enter it again for 90 days, it will revert to showing all posts in that forum as new. It only really matters on quite large forums (i.e. 30,000+ posts in a single forum). You can disable the optimization by setting the value to 0, in which case "new" data will never be forgotten. Doing so won't hurt anything - you can always turn it back on later. The symptom is that the main index page will load quite a bit slower when logged in, particularly if there are a lot of busy forums that you haven't visited in a while.

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Jason Rhinelander
Gossamer Threads
jason@gossamer-threads.com