Function optionlet_time 

pub(crate) fn optionlet_time(
    as_of: NaiveDate,
    end: NaiveDate,
    lag_months: u32,
) -> f64

The time a YoY optionlet’s variance accrues over, in years: ACT/365F(first of the month of (as_of − lag), end − lag) — from the vol’s base date to the coupon’s fixing date, for a non-interpolated YoY optionlet vol whose observation lag is the coupon’s.