How long has the U.S. had access to Time Travel?

That's actually a hard question to answer properly.

Why?

Because UFOs have had time travel since before humans ever were on planet earth and they came here with it then.

But, if you want to answer this question maybe more usefully you could possibly saw that Time Travel became a thing in U.S. technology in 1942 I believe in Philadelphia harbor on board the U.S.S. Eldridge when Einstein and others were trying to make military ships invisible so that U-Boats and planes and enemy ships couldn't see U.S. ships to sink them.

In the process of trying to do this and men getting caught in the metal of the ship in this process and dying often the beginnings of the U.S. Time Travel in the military began.

So, the rudimentary use of Time Travel basically could be said to have begun on board the U.S.S. Eldridge in 1942. But, at that time it wasn't fully understood by earth born humans yet and many men died until it was better understood at this time.

Was time travel used to end World War II more Quickly.

I think the best answer is: "YES!"