Time travel is perhaps the ultimate and most devastating superpower. Any hero (0r villain) Superman - and his method for time travel set the standard for tossing science to the winds.
When he first debuted in Action Comics #1 in 1938, Superman possessed only a small fraction of the powers he has now. He was fast, but couldn't break the sound barrier; strong, but couldn't lift more than a large vehicle; durable, but "...nothing short of a bursting shell can penetrate his skin!" - and though he could leap quite high, he couldn't actually fly. This all changed during the Silver Age of the late 50s and early 60s, when Superman could lift planets, survive a nuclear explosion, and fly faster than the speed of light - which is where the writers at DC Comics got the idea for Superman to break the "time barrier."
The Silver Age Superman believed that, by flying as fast as possible, he could "...burst them time-barrier!" Sometimes this was explained as Superman flying faster than the speed of light (although scientifically, the theory of relativity would only allow forward travel into the future, not into the past). This was usually represented by Superman flying past numbers, signifying important dates and eras throughout history. Realizing that this power made all of Superman's conflicts meaningless, the writers slowly phased it out of continuity...but the same thing couldn't be said for the Flash.
The Flash has multiple methods of time travel: like Superman, he can run faster than light which allows him to travel both forwards and backwards in time. Unlike Superman, his connection to the quasi-mythical Speed Force allows him to avoid typical annoyances like physics, cause and effect, and all other scientific roadblocks as he speeds along. Later on, he would acquire the cosmic treill, a device that utilized Flash's running abilities to travel back in time with more accuracy.
But unlike Superman, the Flash constantly faced consequences of traveling back in time, especially during 2011's Flashpoint event. In an attempt to travel back in time to save his mother, Barry Allen woke up to a world on the brink of war between Amazons and Atlanteens, and Bruce Wayne was killed instead of his father. Eventually the Flash set right what was wrong through the same silly method of time travel as Superman - running incredibly fast. But while Superman's time travel abilities could be used as often as he wanted, the Flash's had significant consequences, which ultimately made for a better story.
