The symmetry of space and time is based on the existence of a constant of proportionality that converts space into time and time into space. This number is a fixed speed for travel under free-flow conditions for the mode of travel. Such a constant makes it arbitrary whether one answers “how long” with a length of space or a length of time, since they are interconvertible.
This constant, called c in the case of light travel in a vacuum, is an absolute within the space-time defined by a mode of travel. Given that the physical universe is the largest observed medium and the mode of electromagnetic transmission is the fastest mode, the free-flow speed of light in a vacuum provides a standard constant of proportionality for space and time in general.
Since a mode of travel may be contextualized to a particular medium or network or conditions, space-time within a particular context is a conventional scheme. This accords with Poincaré’s conventionalism.
With a constant of proportionality between space and time, for every property of one there is a corresponding property of the other. For example, space is three-dimensional and so time is three-dimensional. Time on a trajectory is one-dimensional and so is space within that context. Space is isotropic (the same in all directions) and so time is isotropic. And so forth.