In fact, in special relativity, it is. When calculating the spacetime "distance" between two events, we use something that looks like the Pythagorean theorem, but we must always make sure to put a different sign in front of time than we do space. (If we sum the squares of space, we must subtract the square of time) (see: minkowski metric, really any metric)
Another point of interest would be the matrix representation of the Lorentz transformation (the way to go from one reference frame to another). The matrix "mixes" the values of the time coordinate with the space coordinates, and the space coordinates with each other, but time is mixed differently. (This is a nice one to bring up because its clear if you just look at the matrix.)
This is very significant for people wondering about time's status as a dimension. It is emphatically not the same kind of dimension as space, even if we choose to represent it as a component of the position vector.