Support -0000 as a valid UTC tz in rfc2822

This is a time that is commonly set in some environments, and RFC 5322
explicitly clarifies that we should treat -0000 as UTC[1][2] when interpretting
rfc2822.

Fixes #102

[1]: https://github.com/chronotope/chrono/issues/102#issuecomment-557846931
[2]: https://tools.ietf.org/html/rfc5322#section-3.3

CHANGELOG.md

@@ -10,6 +10,11 @@ Versions with only mechanical changes will be omitted from the following list.
 
 ## next
 
+### Improvements
+
+* Support "negative UTC" in `parse_from_rfc2822` (@quodlibetor #368 reported in
+  #102)
+
 ### Internal improvements
 
 * Use Criterion for benchmarks (@quodlibetor)

src/datetime.rs

@@ -2051,6 +2051,8 @@ mod tests {
 
         assert_eq!(DateTime::parse_from_rfc2822("Wed, 18 Feb 2015 23:16:09 +0000"),
                    Ok(FixedOffset::east(0).ymd(2015, 2, 18).and_hms(23, 16, 9)));
+        assert_eq!(DateTime::parse_from_rfc2822("Wed, 18 Feb 2015 23:16:09 -0000"),
+                   Ok(FixedOffset::east(0).ymd(2015, 2, 18).and_hms(23, 16, 9)));
         assert_eq!(DateTime::parse_from_rfc3339("2015-02-18T23:16:09Z"),
                    Ok(FixedOffset::east(0).ymd(2015, 2, 18).and_hms(23, 16, 9)));
         assert_eq!(DateTime::parse_from_rfc2822("Wed, 18 Feb 2015 23:59:60 +0500"),

src/format/parse.rs

@@ -73,10 +73,6 @@ fn parse_rfc2822<'a>(parsed: &mut Parsed, mut s: &'a str) -> ParseResult<(&'a st
     //   by adding 1900. note that four-or-more-digit years less than 1000
     //   are *never* affected by this rule.
     //
-    // - zone of `-0000` and any unrecognized legacy time zones (including
-    //   *every* one-letter military time zones) are considered "missing",
-    //   in such that we don't actually know what time zone is being used.
-    //
     // - mismatching day-of-week is always an error, which is consistent to
     //   Chrono's own rules.
     //

src/format/scan.rs

@@ -308,11 +308,7 @@ pub fn timezone_offset_2822(s: &str) -> ParseResult<(&str, Option<i32>)> {
         }
     } else {
         let (s_, offset) = timezone_offset(s, |s| Ok(s))?;
-        if offset == 0 && s.starts_with('-') { // -0000 is not same to +0000
-            Ok((s_, None))
-        } else {
-            Ok((s_, Some(offset)))
-        }
+        Ok((s_, Some(offset)))
     }
 }