It's like the whole 'light speed' from star wars, 'warp speed' from star trek, and 'such and such speed' from 'sci-fi fantasy series number 12.'
We don't have the magical maguffin device that can allow us to do this. I think we have the technology available to MAKE a time machine, it's just that we don't have the 'can tap into the fabric of time' maguffin needed.
Example: In Star Trek, all the ships can actually enter 'warp speed' because of things called 'dilithium crystals.' These do not exist in our universe, but almost all the other technology in Star Trek we have a resemblance of now. If we found some kind of 4th-dimension changing phenomenon or dark matter or 'dilithium' in our current universe, we might actually be able to see or move into the past or future.
Theoretically, time travel is possible, but we don't have the... Fuel. We have the car, but not the gasoline. We have the train, but not the coal. We have the gameboy, but no batteries. (Or game for that matter.)
The problem is, does the fuel even exist? We're working on something that isn't in our universe that we know of. Perhaps the LHC can shed some light on that random piece of fuel we need, but right now, we have nothing to push our cars with besides ourselves.