This paper has a pedagogical aim. Indeed, by using the relativistic velocity-addition and Einstein's equivalence principle (EEP), we want to analyse in a simple way the physics of time on a rotating non-inertial frame. We use a didactic approach considering four friends. The first is in the laboratory, the second at rest on the disk at radius r, the third and fourth ride a bicycle in opposite directions but with the same speed with respect to the platform. Since, in General Relativity, time can flow differently at every point, the concept of speed is often ambiguous.