Add RFC822 formats as named constants.

Make sure to print a time zone when formatting even if none is defined.
Add a comment introducing lookupTimezone (not lookupTimeZone).

Fixes isse 577.

R=rsc
CC=golang-dev
https://golang.org/cl/196090

src/pkg/time/format.go

@@ -28,9 +28,12 @@ const (
 	ANSIC    = "Mon Jan _2 15:04:05 2006"
 	UnixDate = "Mon Jan _2 15:04:05 MST 2006"
 	RubyDate = "Mon Jan 02 15:04:05 -0700 2006"
-	RFC850   = "Monday, 02-Jan-06 15:04:05 MST"
-	RFC1123  = "Mon, 02 Jan 2006 15:04:05 MST"
-	Kitchen  = "3:04PM"
+	RFC822   = "02 Jan 06 1504 MST"
+	// RFC822 with Zulu time.
+	RFC822Z = "02 Jan 06 1504 -0700"
+	RFC850  = "Monday, 02-Jan-06 15:04:05 MST"
+	RFC1123 = "Mon, 02 Jan 2006 15:04:05 MST"
+	Kitchen = "3:04PM"
 	// Special case: use Z to get the time zone formatted according to ISO 8601,
 	// which is -0700 or Z for UTC
 	ISO8601 = "2006-01-02T15:04:05Z"
@@ -209,7 +212,7 @@ func (t *Time) Format(layout string) string {
 		case stdISO8601TZ, stdNumTZ:
 			// Ugly special case.  We cheat and take "Z" to mean "the time
 			// zone as formatted for ISO 8601".
-			zone := t.ZoneOffset / 60 // conver to minutes
+			zone := t.ZoneOffset / 60 // convert to minutes
 			if p == stdISO8601TZ && t.ZoneOffset == 0 {
 				p = "Z"
 			} else {
@@ -248,7 +251,21 @@ func (t *Time) Format(layout string) string {
 				p = "am"
 			}
 		case stdTZ:
-			p = t.Zone
+			if t.Zone != "" {
+				p = t.Zone
+			} else {
+				// No time zone known for this time, but we must print one.
+				// Use the -0700 format.
+				zone := t.ZoneOffset / 60 // convert to minutes
+				if zone < 0 {
+					p = "-"
+					zone = -zone
+				} else {
+					p = "+"
+				}
+				p += zeroPad(zone / 60)
+				p += zeroPad(zone % 60)
+			}
 		}
 		b.WriteString(p)
 	}

src/pkg/time/time_test.go

@@ -133,6 +133,7 @@ var formatTests = []FormatTest{
 	FormatTest{"ANSIC", ANSIC, "Thu Feb  4 21:00:57 2010"},
 	FormatTest{"UnixDate", UnixDate, "Thu Feb  4 21:00:57 PST 2010"},
 	FormatTest{"RubyDate", RubyDate, "Thu Feb 04 21:00:57 -0800 2010"},
+	FormatTest{"RFC822", RFC822, "04 Feb 10 2100 PST"},
 	FormatTest{"RFC850", RFC850, "Thursday, 04-Feb-10 21:00:57 PST"},
 	FormatTest{"RFC1123", RFC1123, "Thu, 04 Feb 2010 21:00:57 PST"},
 	FormatTest{"ISO8601", ISO8601, "2010-02-04T21:00:57-0800"},
@@ -286,6 +287,20 @@ func TestParseErrors(t *testing.T) {
 	}
 }
 
+// Check that a time without a Zone still produces a (numeric) time zone
+// when formatted with MST as a requested zone.
+func TestMissingZone(t *testing.T) {
+	time, err := Parse(RubyDate, "Tue Feb 02 16:10:03 -0500 2006")
+	if err != nil {
+		t.Fatal("error parsing date:", err)
+	}
+	expect := "Tue Feb  2 16:10:03 -0500 2006" // -0500 not EST
+	str := time.Format(UnixDate)               // uses MST as its time zone
+	if str != expect {
+		t.Errorf("expected %q got %q", expect, str)
+	}
+}
+
 func BenchmarkSeconds(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		Seconds()

src/pkg/time/zoneinfo.go

@@ -221,6 +221,7 @@ func setupZone() {
 	}
 }
 
+// Look up the correct time zone (daylight savings or not) for the given unix time, in the current location.
 func lookupTimezone(sec int64) (zone string, offset int) {
 	once.Do(setupZone)
 	if len(zones) == 0 {