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 one. In German, there was a whole series of "What If" books -- found in nonfiction 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 historial fiction.

On the other hand, if you have travelers from other times wandering loose, or alt hists whose alternative is the existence of SF elements -- come on in!

For example, in Randall Garrett's Lord Darcy stories, Richard the Lionheart survived that crossbow bolt and founded a strong Plantagenet dynasty. The SF element is that in the Middle Ages magic superceded science, so in this other twentieth century not only is most of western Europe and the Americas in Plantagenet hands, but the technology is barely nineteenth century in many areas, while magic is used for everything from police forensics to food refrigeration.

Examples of true time travel or alternative history SF:

• The Dancer from Atlantis by Poul Anderson
• The Grandfather Paradox by Steven Burgauer
• Seventh Son, Red Prophet, Alvin Prentice, Alvin Journeyman, Heartfire by Orson Scott Card
• Time and Again, About Time, and others by Charles Finney
• Murder and Magic, Too Many Magicians, and Lord Darcy Investigates by Randall Garrett
• The Big Time and other Change War stories by Fritz Leiber, also his Destiny Times Three
• 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
• Time and Again by Clifford D. Simak
• The Time Machine by H. G. Wells
• Nine Princes in Amber and the other Amber novels by Roger Zelazny
• Up the Line by Robert Silverburg
• 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 Entropy Effect by Vonda N. McIntyre
• The Flight of the Horse by Larry Niven
• Up the Line by Robery Silverberg
• Killing Time by Della van Hise
• Fire Watch by Connie Willis

And last, but not least, a supposedly true and extremely well-documented case of actual time travel, An Adventure by A.E. Moberly and E. Jourdain.