Consider %s to have be a timestamp in UTC

src/datetime/tests.rs

@@ -603,6 +603,10 @@ fn test_datetime_parse_from_str() {
         Utc.datetime_from_str("Fri, 09 Aug 2013 23:54:35 GMT", "%a, %d %b %Y %H:%M:%S GMT"),
         Ok(Utc.with_ymd_and_hms(2013, 8, 9, 23, 54, 35).unwrap())
     );
+    assert_eq!(
+        DateTime::parse_from_str("0", "%s").unwrap(),
+        NaiveDateTime::from_timestamp_opt(0, 0).unwrap().and_utc().fixed_offset()
+    );
 }
 
 #[test]

src/format/parsed.rs

@@ -627,7 +627,8 @@ impl Parsed {
     /// plus a time zone offset.
     /// Either way those fields have to be consistent to each other.
     pub fn to_datetime(&self) -> ParseResult<DateTime<FixedOffset>> {
-        let offset = self.offset.ok_or(NOT_ENOUGH)?;
+        // If there is no explicit offset, consider a timestamp value as indication of a UTC value.
+        let offset = self.offset.or(self.timestamp.map(|_| 0)).ok_or(NOT_ENOUGH)?;
         let datetime = self.to_naive_datetime_with_offset(offset)?;
         let offset = FixedOffset::east_opt(offset).ok_or(OUT_OF_RANGE)?;