The End of Eternity (Isaac Asimov, 1955)

The Eternity of the title is an organization and a place which exists outside time. It is staffed by humans (usually male) called Eternals who are recruited from different eras of human history commencing with the twenty-seventh century. The Eternals are capable of traveling “upwhen” and “downwhen” within Eternity and entering the conventional temporal world at almost any point of their choice, apart from a section of the far future which they cannot enter. Collectively they form a corps of Platonic guardians who carry out carefully calculated and planned strategic minimum actions, called Reality Changes, within the temporal world in order to minimise human suffering as integrated over the whole of (future) human history.

A key plot element that emerges quickly as the story unfolds is the relatively static nature of the human societies in the various future centuries, and the repeated failure of space travel in all accessible centuries. We later learn that Twissell (Harlan’s superior) is from "a Century in the 30,000's," yet nothing much is different in that time.