It is some weird property of physics that the overall 4-velocity of any physical thing moving in the universe is normalized to the speed of light, pic. When you are at rest in an inertial frame, your 3-velocity is zero and you 1-velocity is equal to the speed of light. Your one velocity is the rate at which time passes for you. Since everyone is always at rest in their own local frame, everyone always sees their own time go by at the same rate. If you speed up relative to some other rest frame than your own, your 3-velocity will increase in magnitude and your 1-velocity will decrease relative to the other frame to ensure than your 4-velocity is normalized for observers in that frame. This phenomenon is "time dilation" and it explains the twins paradox: when you go to another planet at relativistic speed and then come back, your 1-velocity has been lower due to the requirement for the overall normalization of the 4-velocity being equal to the speed of light. As your 3-velocity approaches the speed of light, your 1-velocity has to approach zero for the overall normalization of the 4-velocoty to always be equal to the speed of light. You can see, taking the limit, that the 1-velocity is zero when the 3-velocity is c. Photons don't accelerate of decelerate. They always move with 3-velocity equal to c. Photons don't experience time dilation. The null interval on which photons move is the limit of relativity where clocks stop ticking. Even in the photon's own frame, the clock can't tick because external observers observe the limit of non-ticking. There would be an irreconcilable paradox (which we say doesn't exist) if we were to say that the photon sees a clock ticking in its own inertial frame. That has nothing to do with dilation however, which would only come if the photon accelerated or decelerated, which it never does because it only moves on the null spacetime interval. (The photon can't have a clock in its inertial frame because clocks have mass.)
