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The concept of a molecular clock is central to any effort essential  to obtain an evolutionary time scale from molecular sequence data. Although traditionally it was the fossil record that provided the authoritative source of information about evolutionary history, increasingly molecular sequence data and their inferred phylogenies are becoming the primary framework employed for inferring the evolutionary history of life on Earth. However despite this fact, inferring the timing of phylogenetic divergences on a geological scale still relies heavily on information synthesized from the fossil record. Early examples of using the fossil record in concert with molecular data may have suggested that such analyses are straightforward, however a recent flurry of new approaches suggests that the science is not yet settled. In this article I will describe the various models of molecular clock, and the various ways they are used in conjunction with data from the fossil record to date the divergence time of molecular phylogenies.