All clocks, periodic and non-periodic, are open dissipative systems driven from thermal equilibrium so that the Helmholtz free energy is increased. In this talk Gerard Milburn discusses the thermodynamic constraints for classical and quantum clocks. He also discusses clocks driven not by work but by information extraction and makes a connection to Rovelli's thermal time hypothesis as a proposed solution to the problem of time in quantum gravity. AIP Theoretical Physics Seminar, August 12, 2021