Add DateTime::fix_offset

src/datetime/mod.rs

@@ -325,6 +325,14 @@ impl<Tz: TimeZone> DateTime<Tz> {
         tz.from_utc_datetime(&self.datetime)
     }
 
+    /// Fix the offset from UTC to its current value, dropping the associated timezone information.
+    /// This it useful for converting a generic `DateTime<Tz: Timezone>` to `DateTime<FixedOffset>`.
+    #[inline]
+    #[must_use]
+    pub fn fixed_offset(&self) -> DateTime<FixedOffset> {
+        self.with_timezone(&self.offset().fix())
+    }
+
     /// Adds given `Duration` to the current date and time.
     ///
     /// Returns `None` when it will result in overflow.

src/datetime/tests.rs

@@ -1920,4 +1920,18 @@ fn test_datetime_local_from_preserves_offset() {
 
     let datetime_fixed: DateTime<FixedOffset> = datetime.into();
     assert_eq!(&offset, datetime_fixed.offset());
+    assert_eq!(datetime.fixed_offset(), datetime_fixed);
+}
+
+#[test]
+fn test_datetime_fixed_offset() {
+    let naivedatetime = NaiveDate::from_ymd_opt(2023, 1, 1).unwrap().and_hms_opt(0, 0, 0).unwrap();
+
+    let datetime = Utc.from_utc_datetime(&naivedatetime);
+    let fixed_utc = FixedOffset::east_opt(0).unwrap();
+    assert_eq!(datetime.fixed_offset(), fixed_utc.from_local_datetime(&naivedatetime).unwrap());
+
+    let fixed_offset = FixedOffset::east_opt(3600).unwrap();
+    let datetime_fixed = fixed_offset.from_local_datetime(&naivedatetime).unwrap();
+    assert_eq!(datetime_fixed.fixed_offset(), datetime_fixed);
 }