Function check_validity_time_tolerance 

pub fn check_validity_time_tolerance(
    now: SystemTime,
    validity: RangeInclusive<SystemTime>,
    pre_tolerance: Duration,
    post_tolerance: Duration,
) -> Result<(), VF>

Like check_validity_time() but with a tolerance to support clock skews.

This function does not use the DirTolerance struct because we want to be agnostic of directories in this context.