I don't need to precisely define it to know that it exists outside of my mind. We have performed experiments that demonstrate that time is real, even if we don't fully understand it. A clock is indeed a concept we created to represent or act as a measurement for what we experience and call time. You can tell that time is real because forces (space and gravity) can act on it as demonstrated by the Hafele-Keating experiment. Time is certainly relative, as predicted by special relativity, but that doesn't mean that time isn't representing a real experience outside of the mind. Try unscrambling an egg or firing a bullet back into a barrel. Our perception of time in the form of Action -> Reaction is clearly real, even if we don't fully understand the mechanisms at work. 

 I don't fully understand gravity either but I can measure it and predict that I will fall if I jump from a bridge. Stepping on an earth scale will yield a different number than a moon scale. It isn't just in my mind either. We have to invent ways to measure and express nature (clocks, scales, math, etc), but that doesn't mean we invented nature.