<p>We consider estimation of hitting-time variance, i.e. the explicit solution in the inverse first-hitting time problem of a continuous martingale to a constant boundary. The nonparametric estimation is based on delta-sequences. We also consider tuning parameter estimation related to the boundary. We characterize feasible statistics induced by central limit theory for the estimation procedure. A numerical simulation corroborates the asymptotic theory. An empirical application to financial data documents that the volatility is periodic at the duration scale. This can be explained by the endogeneity of transaction times.</p>