There aren't enough minutes in the day....

Time travel isn't possible because we don't actually know the scope of what time is.

We base a day on one rotation of the planet.
We've decided that a day consists of 24 hours.

Who decided these things?
That one rotation is one day?
That one day consists of 24 hrs, an hour of minutes, minutes of seconds etc etc?

"Time" as we know it is only a creation of convenience for humans.

What if what we consider a year was only in actual fact, the equivalent of a millisecond?
Any time travel attempts would be seriously out of place by decimal points.

Put into context of the lifespan of the universe, what if the universe had an average lifespan of 80 years?
We assume the universe is either infinite, or just aging at the same speed that we are.
Simply because we as humans appear to age within a year, it doesn't mean that the universe does.

Teleportation would suffer from similar problems, I'd imagine.