BrainBoxLtd on 30 September 2008
I played the 360 version, so I'm not sure if there's a difference, but Timed Challenges were online only challenges that required at very least 2 people. It's a variant of the Freeburn Challenges. If you're hosting a Freeburn Session, hit start, go to challenges and there should be a prompt to change from Freeburn to Timed. (Was X on the 360, which probably means Square on the PS3) If you're a guest in someone else's session, it's up to the host what if any challenges you will do.
