In Minkowski spacetime time is subjective [or more precisely: time is different for every particle/ reference frame]. It is the coordinate time of an observer whose reference frame travels up the $ct$-axis. Before Minkowski, we thought that time was the same for everybody, but now we know that the slightest movement of an observer destroys synchronization with regard to the other observers. Each bus driver lives slower than a prisoner because he is moving, even if the difference is not perceivable. Time is an accumulation of a huge number of relative Minkowski diagrams.
Does that mean that there is no generally applying physical definition of time dimension left? Or is there an absolute dimension of time of the universe in which we are living, and which is independent from the observer?