Prepare for reminding

src/task.rs

@@ -24,6 +24,15 @@ pub enum State {
     Completed { date: jiff::Zoned },
 }
 
+impl State {
+    pub fn due_date_if_open(&self) -> Option<&jiff::Zoned> {
+        match self {
+            State::Open { due } => due.as_ref(),
+            _ => None,
+        }
+    }
+}
+
 #[derive(Debug, Clone)]
 pub struct Recurring {
     pub interval: RecurringInterval,