 Why can't Joe go back to 1960?
A gun is an interesting weapon; it can be hired, of course, and naturally doesn't care who hires it. And at that moment, the universe caved inward upon Joseph Marie Prantera. Joe had been born in Naples and his mother had died in childbirth. Joe Prantera stared at him, said finally, flatly, "What's it all about?" "What is the date, Old Calendar?" "2133," Reston-Farrell said. Joe Prantera had never been exposed to the concept of time travel. Reston-Farrell said, "Suffice to say, you are now one hundred and seventy-three years after the last memory you have." Joe Prantera's mind suddenly reverted to those last memories and his eyes narrowed dangerously.
 Time only moves one way.
 Temple-Tracy destroyed the vortex manipulator.  The time circuits were damaged when they brought Joe into the future.  Temple -Tracy destroyed the time transmitter.  Time only moves one way.