If you ask google for the definition of time, you'll get "the indefinite continued progress of existence and events in the past, present, and future regarded as a whole". But does time even exist? You see, in a normal world, time controls you. You can not control time... If you want to redo the minute of your life even second, you cannot. The phrase "time waits for no man" is very true, time will not even wait for itself. The point of a time machine though is that now YOU control time... If you want to redo that minute or hour or day or year of your life, you can. But how can you control something that isn't even there? I think the problem with time travel isn't that we can't define time, I mean, just because you can't define something doesn't mean you can't control it. I think the problem with time travel, is how can you control something, that isn't truly there?