Twin Paradox, Lorentz Transformation, Relativity.
dx = v(t)*dt dt_1 = γ^(-1)(t)*dt dx = γ(t_1)*v_1 (t_1)*dt_1 dx_1 = dx/γ(t_1)= dx/γ(t) = v_1(t_1)*dt_1 d_1m(t_1) = x(t)*γ^(-1) (t) (the only one in which there are no differentials). This discussion also applies in the event that the body goes back along a further distance d to return to the origin