time-range check
The relation $$ \int_0^T \sigma_L^2(t)dt = \sigma_B^2 T $$ holds, where $ \sigma_L(t) $ is the local volatility at time $ t $ and $ \sigma_B(T) $ is the Black volatility for maturity $ T $. From the above, the formula $$ \sigma_L(t) = \sqrt{\frac{\mathrm{d}}{\mathrm{d}t}\sigma_B^2(t)t} $$ can be deduced which is here implemented.
the latest time for which the curve can return values
the settlementDays used for reference date calculation
date/time conversion
date-range check