= Molecular-clock calibration and rate-time identifiability
{title2=$d=rt$}
A molecular branch length is expected substitutions per site, the product of evolutionary rate and elapsed time. Without external calibration, simultaneous rescaling of rate and time preserves the sequence likelihood. <Fossil> or geological bounds, or informative sampling dates, supply time information; relaxed clocks allow rate variation. Calibration uncertainty, saturation and <gene>/species-tree discordance must be propagated rather than turned into an exact historical date.
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