RUST-1677 Add duration_since methods to DateTime (#417)

src/datetime.rs

@@ -389,6 +389,22 @@ impl crate::DateTime {
         })?;
         Ok(Self::from_time_0_3(odt))
     }
+
+    /// Returns the time elapsed since `earlier`, or `None` if the given `DateTime` is later than
+    /// this one.
+    pub fn checked_duration_since(self, earlier: Self) -> Option<Duration> {
+        if earlier.0 > self.0 {
+            return None;
+        }
+        Some(Duration::from_millis((self.0 - earlier.0) as u64))
+    }
+
+    /// Returns the time elapsed since `earlier`, or a [`Duration`] of zero if the given `DateTime`
+    /// is later than this one.
+    pub fn saturating_duration_since(self, earlier: Self) -> Duration {
+        self.checked_duration_since(earlier)
+            .unwrap_or(Duration::ZERO)
+    }
 }
 
 impl fmt::Debug for crate::DateTime {

src/tests/datetime.rs

@@ -1,3 +1,5 @@
+use std::time::Duration;
+
 use crate::tests::LOCK;
 
 #[test]
@@ -38,3 +40,22 @@ fn datetime_to_rfc3339() {
 fn invalid_datetime_to_rfc3339() {
     assert!(crate::DateTime::MAX.try_to_rfc3339_string().is_err());
 }
+
+#[test]
+fn duration_since() {
+    let _guard = LOCK.run_concurrently();
+
+    let date1 = crate::DateTime::from_millis(100);
+    let date2 = crate::DateTime::from_millis(1000);
+
+    assert_eq!(
+        date2.checked_duration_since(date1),
+        Some(Duration::from_millis(900))
+    );
+    assert_eq!(
+        date2.saturating_duration_since(date1),
+        Duration::from_millis(900)
+    );
+    assert!(date1.checked_duration_since(date2).is_none());
+    assert_eq!(date1.saturating_duration_since(date2), Duration::ZERO);
+}