  We consider a clock 'paradox' framework where an observer leaves an inertial
frame, is accelerated and after an arbitrary trip comes back. We discuss a
simple equation that gives, in the 1+1 dimensional case, an explicit relation
between the time elapsed on the inertial frame and the acceleration measured by
the accelerating observer during the trip.
  A non-closed trip with respect to an inertial frame appears closed with
respect to another suitable inertial frame. Using this observation we define
the differential aging as a function of proper time and show that it is
non-decreasing. The reconstruction problem of special relativity is also
discussed showing that its, at least numerical, solution would allow the
construction of an 'inertial clock'.
