While normally we don't define a velocity for time, there is no reason we can't. Velocity is just distance over time. While it is quite common to express distances in units of time (i.e. light year), it is less common to express time in units of distance. However, if you do so, the definition for time velocity follows. To express time as a unit of distance, simply multiply by the speed of light. Velocities are always relative, so we need a second reference frame to use to measure the time velocity.