Read the Recently Active Threads Faqlet. In particular note:

The time period of the selection is controlled through the "<<" style links at the top of the page. By default the time period is now. The << links go back in time, with more <'s indicating further back, and likewise for >> links but going forward. If a datestamp link appears, then that is the date of the page displayed, which can be used for finding that set of nodes later.