You perceive time based on how long you've been alive. Each infinitesimally small fraction of time you have been alive contributes to you perceiving time as passing by more quickly. I'm not certain, but I think you can get an idea of how you perceive time by finding the square root of your age. You can then figure out how other people perceive time, and how you used to perceive time and will potentially perceive time in the future by taking the square root of the other age. I'm not sure if this is 100% correct, though. Should e be involved in this equation?