physical significance does negative time have in equation relating time and velocity?

Answer 1

Negative time usually just means some event happened before some reference instant, rather than after it.

For example, if you calculate the time it takes for a cannonball to reach ground level after being fired off a cliff, the equations will give you two answers for "t"; one positive and the other negative. A value of "t=-2" means that the cannonball's trajectory (if followed _backwards_), would also have put it at ground level 2 seconds _before_ it was actually fired.

In cases like this, one often rejects the negative value because it doesn't make sense with the details of the problem.