Add more DateTime.into tests

More tests for combinations of `DateTime::into`.

Follow-up to Pull Request chronotope#271.

src/datetime/mod.rs

@@ -1530,6 +1530,61 @@ where
     }
 }
 
+#[test]
+fn test_add_sub_months() {
+    let utc_dt = Utc.with_ymd_and_hms(2018, 9, 5, 23, 58, 0).unwrap();
+    assert_eq!(utc_dt + Months::new(15), Utc.with_ymd_and_hms(2019, 12, 5, 23, 58, 0).unwrap());
+
+    let utc_dt = Utc.with_ymd_and_hms(2020, 1, 31, 23, 58, 0).unwrap();
+    assert_eq!(utc_dt + Months::new(1), Utc.with_ymd_and_hms(2020, 2, 29, 23, 58, 0).unwrap());
+    assert_eq!(utc_dt + Months::new(2), Utc.with_ymd_and_hms(2020, 3, 31, 23, 58, 0).unwrap());
+
+    let utc_dt = Utc.with_ymd_and_hms(2018, 9, 5, 23, 58, 0).unwrap();
+    assert_eq!(utc_dt - Months::new(15), Utc.with_ymd_and_hms(2017, 6, 5, 23, 58, 0).unwrap());
+
+    let utc_dt = Utc.with_ymd_and_hms(2020, 3, 31, 23, 58, 0).unwrap();
+    assert_eq!(utc_dt - Months::new(1), Utc.with_ymd_and_hms(2020, 2, 29, 23, 58, 0).unwrap());
+    assert_eq!(utc_dt - Months::new(2), Utc.with_ymd_and_hms(2020, 1, 31, 23, 58, 0).unwrap());
+}
+
+#[test]
+fn test_auto_conversion_fixedoffset_into_utc() {
+    let utc_dt = Utc.with_ymd_and_hms(2018, 9, 5, 23, 58, 0).unwrap();
+    let cdt_dt = FixedOffset::west_opt(5 * 60 * 60)
+        .unwrap()
+        .with_ymd_and_hms(2018, 9, 5, 18, 58, 0)
+        .unwrap();
+    let utc_dt2: DateTime<Utc> = cdt_dt.into();
+    assert_eq!(utc_dt, utc_dt2);
+}
+
+#[test]
+fn test_auto_conversion_utc_into_fixedoffset() {
+    let utc_dt = Utc.with_ymd_and_hms(2018, 9, 5, 23, 58, 0).unwrap();
+    let cdt_dt = FixedOffset::west_opt(5 * 60 * 60)
+        .unwrap()
+        .with_ymd_and_hms(2018, 9, 5, 18, 58, 0)
+        .unwrap();
+    let cdt_dt2: DateTime<FixedOffset> = utc_dt.into();
+    assert_eq!(cdt_dt, cdt_dt2);
+}
+
+#[cfg(feature = "clock")]
+#[test]
+fn test_auto_conversion_local_into_utc() {
+    let loc_dt = Local.with_ymd_and_hms(2020, 1, 2, 3, 4, 5).unwrap();
+    let utc_dt: DateTime<Utc> = loc_dt.into();
+    assert_eq!(utc_dt, loc_dt);
+}
+
+#[cfg(feature = "clock")]
+#[test]
+fn test_auto_conversion_utc_into_local() {
+    let utc_dt = Utc.with_ymd_and_hms(2020, 1, 2, 3, 4, 5).unwrap();
+    let loc_dt: DateTime<Local> = utc_dt.into();
+    assert_eq!(utc_dt, loc_dt);
+}
+
 #[cfg(all(test, any(feature = "rustc-serialize", feature = "serde")))]
 fn test_encodable_json<FUtc, FFixed, E>(to_string_utc: FUtc, to_string_fixed: FFixed)
 where