Bad science aside, the bigger objection might be that turning back time is a massive deus ex machina - a narrative cheat. Moreover, a major feat like that really should have had a bit of foreshadowing - some hint (at least) that time-travel was among Supes arsenal of powers. Otherwise, it comes across as a wtf? moment. Actually according to certain wormhole time machine models, you could travel backward in time - but no earlier than the date the machine was invented. So for example, the 1985 DeLorean could go to 2015 and then back to 1985. But 1955 is forbidden.