Any era. The default setting for our game is 2371 (post-Next Generation, Deep Space 9 season 3, Voyager season 1) but this doesn’t stop you from following in the footsteps of Captain Archer or Captain Kirk in the 22nd and 23rd centuries, or making your own adventures after Star Trek: Nemesis. The rules themselves do not limit your ability to play in any part of the Star Trek timeline.