Round seconds to nearest minute when serializing offsets

src/datetime/mod.rs

@@ -1447,7 +1447,9 @@ where
             &FixedOffset::east_opt(3650).unwrap().with_ymd_and_hms(2014, 7, 24, 12, 34, 6).unwrap()
         )
         .ok(),
-        Some(r#""2014-07-24T12:34:06+01:00:50""#.into())
+        // Seconds are not allowed by RFC 3339, offset should be rounded to the nearest minute.
+        // In this case +01:01
+        Some(r#""2014-07-24T12:34:06+01:01""#.into())
     );
 }
 

src/datetime/serde.rs

@@ -41,14 +41,23 @@ impl<Tz: TimeZone> ser::Serialize for DateTime<Tz> {
 
         impl<'a, Tz: TimeZone> fmt::Display for DisplayModifiedRfc3339<'a, Tz> {
             fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-                // Reuse `Debug` impls, which is faster.
+                // Don't use DateTime::to_rfc3339 because we want to use a more compact
+                // representation (`Z`) for UTC datetimes.
+
+                // Reuse the `Debug` impl of `NaiveDateTime`.
                 Debug::fmt(&self.inner.naive_local(), f)?;
                 let offset = (*self.inner.offset()).fix();
-                // Choose a more compact representation (`Z`) for UTC datetimes.
                 if Some(offset) == FixedOffset::east_opt(0) {
                     f.write_char('Z')
                 } else {
-                    Debug::fmt(&offset, f)
+                    // We do not reuse the `Debug` implementation of `FixedOffset` because that may
+                    // contain seconds, which are not allowed by RFC 3339.
+                    let offset = offset.local_minus_utc();
+                    let (sign, offset) = if offset < 0 { ('-', -offset) } else { ('+', offset) };
+                    let mins = (offset + 59) / 60;
+                    let hour = mins / 60;
+                    let min = mins % 60;
+                    write!(f, "{}{:02}:{:02}", sign, hour, min)
                 }
             }
         }