  This paper puts forward a broad critical analysis of the concept of physical
time. Clock effect is conceived as a consequence of the variation of the
gravitational or pseudo gravitational potential, and it is remarked that only
some real clocks measure durations in agreement with the predictions of general
relativity. A probable disagreement is expected between radioactive and atomic
clocks, in the light of Rovelli's thermal time hypothesis. According to the
recent contributions by Rugh and Zinkernagel, the relationship between time and
clocks is investigated in order to found on a physical basis the concept of
cosmic time. In the conclusive section is argued the impossibility of reducing
thermal time to relativistic time.
