In general, when I think of movement through space, I think of this:
$$\frac{dx}{dt}$$
But in special relativity, we also have a concept of relative duration, which means that $t$ must have a rate of change, but with respect to what?
$$\frac{dt}{d?}$$
This is a near duplicate of What is the speed of time? and Terry has given a comprehensive answer there. However there is one point that wasn't made in the previous question.
In special and general relativity there is an invarient called proper time, $\tau$, which is is the time measured by a freely moving observer, and it is perfectly reasonable to ask what $dt/d\tau$ is provided you're clear what you mean by $t$. For a freely moving observer $dt/d\tau$ is always one, but this won't be so for other observers. For example if we watch someone falling into a black hole we will see their time slow as they approach the event horizon. So if by $t$ we mean our (Schwarzschild) co-ordinate time then $dt/d\tau$ is not unity.
