  We consider models of nucleotidic substitution processes where the rate of
substitution at a given site depends on the state of its neighbours. For a wide
class of such nonreversible models, we show how to compute consistent,
mathematically exact, estimators of the time elapsed between aligned sequences,
for an ancestral sequence and a present one, and also for two present
sequences. In both cases, we provide asymptotic confidence intervals, valid for
nucleotidic sequences of finite length. We compute explicit formulas for the
estimators and for their confidence intervals in the simplest nontrivial case,
the Jukes-Cantor model with CpG influence.
