  All clocks, classical or quantum, are open non equilibrium irreversible
systems subject to the constraints of thermodynamics. Using examples I show
that these constraints necessarily limit the performance of clocks and that
good clocks require large energy dissipation. For periodic clocks, operating on
a limit cycle, this is a consequence of phase diffusion. It is also true for
non periodic clocks (for example, radio carbon dating) but due to telegraph
noise not to phase diffusion. In this case a key role is played by accurate
measurements that decrease entropy, thereby raising the free energy of the
clock, and requires access to a low entropy reservoir. In the quantum case, for
which thermal noise is replaced by quantum noise (spontaneous emission or
tunnelling), measurement plays an essential role for both periodic and non
periodic clocks. The paper concludes with a discussion of the Tolman relations
and Rovelli's thermal time hypothesis in terms of clock thermodynamics.

Quantum Physics;Classical Physics;History and Philosophy of Physics