I don't like the save loading idea, I think that was a joke suggestion anyway. That could really piss people off
But I do like the moving back and forth in time, if done right that could be really cool. Question is how exactly it would work
If jumping into the future:
* would the player's hunger be affected?
* would food they're carrying spoil?
* would things that are on fire burn up?
if jumping into the past:
* would the player teleport back to where they were at that point in time?
* would mobs that had been killed during that time come back to life?
Being able to track and restore their state would mean it would have to be limited to a certain number of steps... but I think that's better anyway. It would be much more clear to the player that they're jumping back in time - I'm thinking something like Dr. Strange fighting Dormammu where they get frustrated having to fight him over and over again (but are also forced to try something new each time, since the pseudorandom seed would be the same, repeating the same actions would result in the exact same outcome). How cool would it be if we could actually animate things "rewinding"?? And if you encounter him above ground being able to see the light change as the sun rises/sets.
Also, an idea to keep him from being overpowered - his time traveling ability could be limited to a certain number of uses, it could even be a "staff of time travel" that he uses, and then if the player manages to kill him before he uses all the charges, they may even be able to time travel by their own choice!