As per my understanding time has no meaning unless it passes... So time is actually the interval between two moments. It is like "length" of an object, which is the distance between two ends of an object/points. It cant be negative. A point object assumed in physics theories has zero length. But unlike LENGTH, the observed time interval between two events could be different for two people, which depends on the speed with which they are moving. Two observer, one stationary wrt earth and other moving at speed lets say at c/2, will observe different readings on their watch between events A and B. Both observer can disagree for the time interval between two events but they will agree on the order in which the two events occur.
But some scientist believe that if you travel faster than light, time will run in negative direction.
lets take an example from the book "A Brief History of Time".
Lets take first event A to occur on earth in 2012 and second event B on alpha century in 2017. the time is as per earth's time. As alpha centuary is around 4 light years form us, so the event which we see in 2017 must have occured in 2013. Now for an observer on earth, he will see event A occured first and his watch will agree with his statement. The second observer can move much faster than light, So he start his space ship when event A occured and move much faster than light to reach Alpha Century just in time to observer event B. But as his watch will show a time before 2012, lets say 2010, he will conclude that event B occured before event A. Thus both observer will not agree on the order of the event.
The above example does not sound convincing to me. See the observer saw the first event, then started his space ship and then saw the second event. now because his watch/calender shows an earlier time when he was at event B, does not mean he was event B before event A. The problem here is that we have assigned a direction to time, which is merely a interval between duration. its a scalar quantity. When one travels at a speed more than light our biological clocks would reverse. We would grow younger from older... The notion that our young form come earlier than older form is only true wrt to our present situation in which we are not moving at > C.
So it is wrong to assign a sign to time. IT IS A RELATIVE QUANTITY AND IT MEASURES THE INTERVAL BETWEEN TWO EVENTS.
