We are exploring multiple methods for estimating divergence times using mitochondrial sequences. There seems to be a common approach that converts % sequence divergence into calendar years (very dodgy of course!), but regardless of how how problematic this approach is, I can't find any arguments that says why sequence divergence should be linked to calendar years rather than the number of generations. I always thought that most mutations arose during meiotic events, rather than as a pure function of time. The studies I've read don't explain their rationale. Do they perhaps just assume one generation per year? Which would be a weird kind of assumption.