Science Fiction In The Past

How can science fiction be set in historical or even in prehistorical periods? At least three sf themes can be exported to the past: alien contact, immortality and time travel. Needless to say, Poul Anderson presents all three. The alien contact is in The High Crusade but I want to concentrate on the other two themes.

The Boat Of A Million Years features mutant immortals living through history;
There Will Be Time features mutant time travelers traveling through history;
The Corridors Of Time and The Dancer From Atlantis feature technological time travelers traveling through history;
the Time Patrol series features technological immortal time travelers traveling through history.

By "immortal,"I mean "immune to illness and old age," not "physically indestructible," thus not literally immortal. Nevertheless, the term "immortal" is used as a convenient description for the unaging characters in Boat. Although this point is underplayed, the Time Patrollers are equally unaging. We see Everard and his colleagues only within a few years or decades of their personal lifespans after their recruitment to the Patrol but they should meet others who are considerably older.

As we age, each extra year is a smaller fraction of the total to date so that time seems to "pass more quickly" with age. We state our age in years but do not usually calculate it to the nearest month. Someone who lives for centuries will no doubt regard decades the way we regard years. A single year will be next to nothing on such a timescale. Thus, mere longevity would change our perspective on time. Combining longevity with time travel would transform that perspective out of all recognition.