(from the book):
From ancient humanoids of four billion B.C. to the Kelvan Invasion of the 123rd century, all time stands open before you. All Our Yesterdays is the complete guide to time travel in the world of Star Trek roleplaying, covering everything from time travel techniques such as the slingshot effect and Borg temporal vortices to the rifts in space-time which snare unwary travellers.
Cross your 23rd-century Star Trek Roleplaying Game characters over into your The Next Generation or Deep Space Nine series (or vice versa), or run a pure time travel series. The first of our Expanded Universe sourcebooks combines the entire cosmos of Star Trek roleplaying opportunities.