Add missing test coverage for the previous commit's changes.

src/datetime/tests.rs

@@ -1171,6 +1171,13 @@ fn test_from_system_time() {
         )
         .unwrap()
     );
+    assert_eq!(
+        DateTime::<Utc>::try_from(UNIX_EPOCH - Duration::new(999_999_999, 0)).unwrap(),
+        Utc.from_local_datetime(
+            &NaiveDate::from_ymd_opt(1938, 4, 24).unwrap().and_hms_nano_opt(22, 13, 21, 0).unwrap()
+        )
+        .unwrap()
+    );
 
     // DateTime<Utc> -> SystemTime
     assert_eq!(SystemTime::try_from(epoch).unwrap(), UNIX_EPOCH);
@@ -1204,7 +1211,10 @@ fn test_from_system_time() {
     // DateTime<any tz> -> SystemTime (via `with_timezone`)
     #[cfg(feature = "clock")]
     {
-        assert_eq!(SystemTime::try_from(epoch.with_timezone(&Local)).unwrap(), UNIX_EPOCH);
+        let local_at_epoch = epoch.with_timezone(&Local);
+        let local_systemtime_at_epoch = SystemTime::try_from(local_at_epoch).unwrap();
+        assert_eq!(local_systemtime_at_epoch, UNIX_EPOCH);
+        assert_eq!(DateTime::<Local>::try_from(local_systemtime_at_epoch).unwrap(), local_at_epoch);
     }
     assert_eq!(
         SystemTime::try_from(epoch.with_timezone(&FixedOffset::east(32400).unwrap())).unwrap(),