How is time defined in astronomy? I was amazed when my teacher told me that in space, time is define as a coordinate. It means we can define something  with a four coordinate system with "negative time" $-t$ as easily as we can say $-x$. Assuming that we can, what will this suggest?

Edit:) yesterday I saw so many answer on it, create one more doubt in me, that how do I define a origin in space, because there nothing to which I compare, suppose I have two events A and B, how can I say A occur before B, if I say this, from which reference I am comparing it....
$t$ signifies time; see the Wikipedia article for spacetime, and then the subsection for 4-vectors. 

The basics are pretty natural to understand. Suppose something happens, an event, like an apple falling off of a tree. 

In order to tell someone else about it you need the three space coordinates $x, y, z$ and the time coordinate $t$. Without all four, you won't be able to see it happen.



update: The question was clarified to focus on the meaning of "negative time" or $-t$. That's no big deal. There's no problem with $-x$ for example. We don't think of that as negative distance or negative space. It's just a coordinate relative to some origin $(0, 0, 0)$, and we can put that anywhere, arbitrarily. 

So you should really thing of the time coordinate as $t-t_0$ where $t_0$ is some arbitrary origin in time. It might be midnight yesterday, or midnight next Tuesday. That would make $t_{Now}$ positive in the first case and negative in the second case, but either way it's still Now.