Macro use_signed_duration 

macro_rules! use_signed_duration {
    (
        $main_trait:ident $internal_trait:ident =>
        {
            $ty:ty =>
            $({
                $format:ty, $strictness:ty =>
                $($tbound:ident: $bound:ident $(,)?)*
            })*
        }
    ) => { ... };
    (
        $( $main_trait:ident $internal_trait:ident, )+ => $rest:tt
    ) => { ... };
}