Time travel stories are among the core motifs of SF, but is it science fiction or fantasy? You can have impressive physics lectures on tachyons, or put on a magic ring, and the characters get around equally well, independent of the set-dressing. Besides plain, ordinary, mundane travel forwards and backwards, there’s cross-timing, too, into variations of our own or other worlds.
From this rose alternate or alternative history stories. One of the first of these was “What If Lee Had Lost At Gettysburg” by (Sir) Winston Churchill, written from the point of view of an historian in a world where the CSA had won that battle. In German, there was a whole series of “What If” books — found in the nonfiction section of the library in English translation — with articles by historians as to how history might have gone.
Alternative history, therefore, is definitely part of SF. However, for the purposes of a writing workshop, alternative history without time travel or other speculative elements is not being supported here. That’s because those sort of “alt hists” are not written like SF: they are written like historical novels. Examples of this not-SF are A Traveller in Time by Alison Uttley, and Outlander by Diana Gabaldon. Considering the number of historical novels, romances, and mysteries that badly distort the culture and history of their purported setting, it’s getting hard to tell the alt hists from the plain historical fiction.
On the other hand, if you have travelers from other times wandering loose, or alt hists whose alternative is the existence of specfi elements — come on in!
Examples of true time travel stories (check out the bookstore for many more):
The Depths of Time by Roger McBride Allen
The Dancer from Atlantis by Poul Anderson
The Time Patrol series by Poul Anderson
The Grandfather Paradox by Steven Burgauer
Time and Again, About Time, and others by Charles Finney
The Ivanhoe Gambit, The Pimpernel Plot, The Six-Gun Solution, The Dracula Caper, The Khyber Connection, The Nautilus Sanction, The Timekeeper Conspiracy, The Hellfire Rebellion, The Zenda Vendetta, by Simon Hawke
The Big Time and other Change War stories by Fritz Leiber, also his Destiny Times Three
The Entropy Effect by Vonda N. McIntyre
The Flight of the Horse by Larry Niven
The Crossroads of Time, Quest Crosstime, The Time Traders, Key Out of Time, Wraiths of Time, Operation Time Search, and lots of others by Andre Norton
Up the Line by Robert Silverberg
Time and Again by Clifford D. Simak
Killing Time by Della van Hise
The Time Machine by H. G. Wells
Fire Watch by Connie Willis
To Say Nothing of the Dog by Connie Willis
Nine Princes in Amber and the other Amber novels by Roger Zelazny