Time is a direction in which you move and the phenomenon of relativistic time-dilation indicates that space and time are identical because the time-dilation phenomenon is the consequence of the summation of the movement vectors through time and space where the sum of the vectors is always the speed of light 'c'. Thus, if you are stationary in space you are traveling at 'c' in the time direction but as you start to move through space the rate at which you move through time must decrease to maintain the summed vector value of 'c'.

As your spatial vector approaches 'c' your temporal vector approaches zero, but this has an important effect on energy conservation, which is dependent upon the temporal vector speed, so as your spatial vector approaches 'c' and your temporal vector approaches zero, the energy values approach infinity, imposing an upper limit that cannot be exceeded as it would require infinite energy.

If you plot the curve of the temporal vector against the spatial vector you get a quadrant of a circle but if you try to extend the curve past the point where the temporal vector becomes zero i.e. at the point where the spatial vector = 'c', you end up needing to find the square root of a negative number, which you can't, to sum the vectors, and which may be why time is unidirectional.