Why compare what an optical mirror does, with what certain symmetries of nature do to particle-antiparticle pairs? Mathematically, reflection symmetries are exact in that the changes in parity always preserve or fix, an orientation. For example, your mirror reflection doesn't change the up direction, your reflection only changes left to right. Mirror reflection is equivalent to a parity transform because direction of motion is reversed, and motion is in one dimension. So apparently, CPT is a theorem that says the symmetry of time (except time is a symmetry), is equivalent to the combined symmetries of C and P. So that a proper *reflection* of a clock is a clock of antimatter with hands moving in the opposite sense. Is the antimatter clock showing negative time, though?