Function libimagtodo::store::date_sanity_check

pub fn date_sanity_check(
    scheduled: Option<&NaiveDateTime>, 
    hidden: Option<&NaiveDateTime>, 
    due: Option<&NaiveDateTime>
) -> RResult<(), String>

Perform a sanity check on the scheduled/hidden/due dates

This function returns a String as error, which can be shown as a warning to the user or as an error.