The Time Wardens

In Poul Anderson's The Corridors Of Time (London, 1968), Wardens and Rangers using time corridors are based about 2000 years after 1964 and a third group, "the time wardens," using conventional time machines, is based about 1000 years later again. Only five of over two hundred pages are set in the time warden period.

A torpedo-shaped "time warden" vehicle enclosing its passengers has to fly to the point where it will transit time whereas a Time Patrol timecycle carrying one or two riders can fly and hover but can also jump to any other place and time. Thus, time wardens use enclosed flying/time machines whereas the Time Patrol uses unenclosed flying/space-time machines.

Warden society has villages, palace-temples and dispersed factories. Rangers have large, crowded metropolises. The time wardens have green land surrounding large, clean, tasteful cities with parks. Warden and Ranger technology is concentrated on warfare whereas time wardens have half-mile long interstellar liners.

In The Corridors Of Time, the time warden period transcends the Warden-Ranger conflict;
in There Will Be Time, the Star Masters period follows the victory of Havig's group over the Eyrie;
in the Time Patrol series, the Danellian period transcends the conflicts of human history.