Function due 

fn due(
    fired: &[TaskChase],
    rung: ChaseRung,
    id: &TaskId,
    since: Option<SystemTime>,
    cadence: u64,
    now: SystemTime,
) -> bool

Whether a recurring rung has waited out its cadence: it is due when nothing fired since since (the instant its state appeared, where the history says), or when the newest firing since then is at least cadence seconds old.