net/mail: more liberal parsing of Date headers.

Fixes #3639.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6243045

src/pkg/net/mail/message.go

@@ -69,11 +69,12 @@ var dateLayouts []string
 func init() {
 	// Generate layouts based on RFC 5322, section 3.3.
 
-	dows := [...]string{"", "Mon, "}     // day-of-week
-	days := [...]string{"2", "02"}       // day = 1*2DIGIT
-	years := [...]string{"2006", "06"}   // year = 4*DIGIT / 2*DIGIT
-	seconds := [...]string{":05", ""}    // second
-	zones := [...]string{"-0700", "MST"} // zone = (("+" / "-") 4DIGIT) / "GMT" / ...
+	dows := [...]string{"", "Mon, "}   // day-of-week
+	days := [...]string{"2", "02"}     // day = 1*2DIGIT
+	years := [...]string{"2006", "06"} // year = 4*DIGIT / 2*DIGIT
+	seconds := [...]string{":05", ""}  // second
+	// "-0700 (MST)" is not in RFC 5322, but is common.
+	zones := [...]string{"-0700", "MST", "-0700 (MST)"} // zone = (("+" / "-") 4DIGIT) / "GMT" / ...
 
 	for _, dow := range dows {
 		for _, day := range days {

src/pkg/net/mail/message_test.go

@@ -95,6 +95,11 @@ func TestDateParsing(t *testing.T) {
 			"21 Nov 97 09:55:06 GMT",
 			time.Date(1997, 11, 21, 9, 55, 6, 0, time.FixedZone("GMT", 0)),
 		},
+		// Commonly found format not specified by RFC 5322.
+		{
+			"Fri, 21 Nov 1997 09:55:06 -0600 (MDT)",
+			time.Date(1997, 11, 21, 9, 55, 6, 0, time.FixedZone("", -6*60*60)),
+		},
 	}
 	for _, test := range tests {
 		hdr := Header{