Time travel stories can fall under at least two distinct genres: science fiction, if the emphasis is on technology or the future, or historical fiction, if the emphasis is on going back to the past. Make it clear in your book blurb which one fits since the two types tend to have entirely different audiences.
The mechanics of time travel and how it might be accomplished are definitely science fiction’s domain, along with speculations about the future and where the world is headed. On the other hand, if a character stumbles across a portal and is simply transported back in time, the emphasis will be on what he encounters. This could include culture shock as well as problems related to how he or she might be able to return to their own time.
A fan of hard science fiction is likely to be disappointed in the latter case whereas a history buff would love it, provided the historical details are accurate. Identifying the story strictly as “time travel” is not specific enough for a potential reader to know, making the book blurb critically important.