More to come yesterday... or maybe tomorrow?
By exceeding the speed of light, and over taking it, it is possible to view images from the past as in "Superman Returns to Krypton".
Time Travel itself can be achieved by travelling at "Super-Speed" and exceeding the speed of light. Through this method, Superman, both as Superboy and later as Superman, have broken "through the time barrier" and arrived either in the past or in the future. The following effects have been observed in the chronicles:
Rule Number One: it is impossible to change events in the past. Proven time and time again by various members of the Superman family.
Rule Number Two: it is impossible to change events in the future as when Supergirl discovered when she appeared in the future and saw the "death" of Jimmy Olsen and tried to prevent it in "present day".
Rule Number Three: It is possible to be the cause or inspiration for something in the past that was ordained to be as a result of a super being's time trip. Various examples throughout the Chronicles of Superman.
The Legion of Super-Heroes travels by Time-Bubble or Time Cube. In the far flung future of tomorrow, Superman will visit the Adult Legion of Super-Heroes by Time Lanes because time trips will be so commonplace.
Although throughout most of the chronicles Superman travels through time by travelling faster than the speed of light, by October 1980, the texts slightly alter the explanation to the effect that he must simultaneously rotate in a "counter clockwise motion" as he "approaches the speed of light" in order to break through the time barrier (NSB No. 10, Oct 1980: "The Town That Time Remembered!")
In addition, Superboy has theorized, following Albert Einstein, that time is circular and that by travelling back in time he can eventually return to the present after traversing the future (NSB No.38, Feb 1983: "The Day That Lasted Forever!").