Do not use Offset's Debug impl when serializing DateTime

src/datetime/mod.rs

@@ -1418,7 +1418,9 @@ 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 `+01:00:50` becomes `+01:01`
+        Some(r#""2014-07-24T12:34:06+01:01""#.into())
     );
 }
 

src/datetime/rustc_serialize.rs

@@ -1,6 +1,6 @@
 #![cfg_attr(docsrs, doc(cfg(feature = "rustc-serialize")))]
 
-use super::DateTime;
+use super::{DateTime, SecondsFormat};
 #[cfg(feature = "clock")]
 use crate::offset::Local;
 use crate::offset::{FixedOffset, LocalResult, TimeZone, Utc};
@@ -10,7 +10,7 @@ use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
 
 impl<Tz: TimeZone> Encodable for DateTime<Tz> {
     fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
-        format!("{:?}", self).encode(s)
+        self.to_rfc3339_opts(SecondsFormat::AutoSi, true).encode(s)
     }
 }
 

src/datetime/serde.rs

@@ -3,11 +3,12 @@
 use core::fmt;
 use serde::{de, ser};
 
-use super::DateTime;
+use super::{DateTime, SecondsFormat};
+use crate::format::write_rfc3339;
 use crate::naive::datetime::serde::serde_from;
 #[cfg(feature = "clock")]
 use crate::offset::Local;
-use crate::offset::{FixedOffset, TimeZone, Utc};
+use crate::offset::{FixedOffset, Offset, TimeZone, Utc};
 
 #[doc(hidden)]
 #[derive(Debug)]
@@ -25,7 +26,7 @@ pub struct MicroSecondsTimestampVisitor;
 #[derive(Debug)]
 pub struct MilliSecondsTimestampVisitor;
 
-/// Serialize into a rfc3339 time string
+/// Serialize into an ISO 8601 formatted string.
 ///
 /// See [the `serde` module](./serde/index.html) for alternate
 /// serializations.
@@ -34,18 +35,19 @@ impl<Tz: TimeZone> ser::Serialize for DateTime<Tz> {
     where
         S: ser::Serializer,
     {
-        struct FormatWrapped<'a, D: 'a> {
-            inner: &'a D,
+        struct FormatIso8601<'a, Tz: TimeZone> {
+            inner: &'a DateTime<Tz>,
         }
 
-        impl<'a, D: fmt::Debug> fmt::Display for FormatWrapped<'a, D> {
+        impl<'a, Tz: TimeZone> fmt::Display for FormatIso8601<'a, Tz> {
             fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-                self.inner.fmt(f)
+                let naive = self.inner.naive_local();
+                let offset = self.inner.offset.fix();
+                write_rfc3339(f, naive, offset, SecondsFormat::AutoSi, true)
             }
         }
 
-        // Debug formatting is correct RFC3339, and it allows Zulu.
-        serializer.collect_str(&FormatWrapped { inner: &self })
+        serializer.collect_str(&FormatIso8601 { inner: self })
     }
 }
 
@@ -1151,11 +1153,11 @@ pub mod ts_seconds_option {
 
 #[cfg(test)]
 mod tests {
-    use core::fmt;
     #[cfg(feature = "clock")]
     use crate::datetime::test_decodable_json;
     use crate::datetime::test_encodable_json;
     use crate::{DateTime, FixedOffset, TimeZone, Utc};
+    use core::fmt;
 
     #[test]
     fn test_serde_serialize() {