Backwards time travel is physically impossible. You would need to travel at truly negative speed to achieve it, but the magnitude of speed is always positive.
Thinking about it, it's even more complicated than that. You're right that a negative speed would not work since the square of a negative number is positive anyway. But a speed higher than c would result in a negative square root, so no real solution. Also, traveling at exactly the speed of light would divide by zero lol
But maybe our formula is wrong. Or it's just impossible, who knows?