The Holont: Second Potentiality and Past And Future.
Thus, we have reviewed inter-temporal communicators in works by Blish, Anderson and Benford. (Blish has four interstellar communicators: the FTL ultraphone, the instantaneous Circon radio and two versions of the Dirac transmitter although only the Dirac-with-beep is intertemporal:
Circon, reaching around the continuum, detects and replies to radio
waves currently transmitted light years away whereas the Dirac
communicator controls the placement of an electron in the circuits of
another Dirac communicator by controlling the frequency and path of a
positron moving through a crystal lattice accompanied by de Broglie
waves which are transforms of the waves of the electron so that a
message is received by amplifying the bursts and reading the signal.
Dirac is not radio;
its effect is simultaneous, not wave-like;
CirCon and Dirac are not two names for the same device.
-copied from here.)
Now, from interstellar and inter-temporal communicators to time travel:
HG Wells' Time Traveler learns the ultimate fates of man and life on Earth;
Poul Anderson's Jack Havig learns that there will be a nuclear war, then lives after it;
although Havig flees back downtime from his first sight of the later Star Masters civilization, he then learns that he and his companions build that civilization;
in a different kind of timeline, Time Patrolman Carl Farness learns that a harrowing experience in his personal future is already part of the historical past which he can, but must not, change;
to Time Patrollers, all times are past or simultaneous but they must still experience those that they visit in a subjective order;
after any futureward time journey, Patrollers might arrive in an altered timeline where they do not know what is happening.