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.