improve debuggability

more Debug-derives, trace/verbose, …

src/date.rs

@@ -10,6 +10,7 @@ use std::collections::BTreeSet;
 use std::error::Error;
 
 /// Defines a day within the month. Negative numbers count from the end.
+#[derive(Debug, Clone, Copy)]
 pub enum DaySpec {
     /// nth day, no matter the weekday
     DayOfMonth(i32),