time: add support for parsing timezones denoted by sign and offset

IANA Zoneinfo does not provide names for all timezones. Some are denoted
by a sign and an offset only. E.g: Europe/Turkey is currently +03 or
America/La_Paz which is -04 (https://data.iana.org/time-zones/releases/tzdata2018c.tar.gz)

Fixes #24071

Change-Id: I9c230a719945e1263c5b52bab82084d22861be3e
Reviewed-on: https://go-review.googlesource.com/98157Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/time/format.go

@@ -1117,6 +1117,11 @@ func parseTimeZone(value string) (length int, ok bool) {
 		length = parseGMT(value)
 		return length, true
 	}
+	// Special Case 3: Some time zones are not named, but have +/-00 format
+	if value[0] == '+' || value[0] == '-' {
+		length = parseSignedOffset(value)
+		return length, true
+	}
 	// How many upper-case letters are there? Need at least three, at most five.
 	var nUpper int
 	for nUpper = 0; nUpper < 6; nUpper++ {
@@ -1153,21 +1158,29 @@ func parseGMT(value string) int {
 	if len(value) == 0 {
 		return 3
 	}
+
+	return 3 + parseSignedOffset(value)
+}
+
+// parseSignedOffset parses a signed timezone offset (e.g. "+03" or "-04").
+// The function checks for a signed number in the range -14 through +12 excluding zero.
+// Returns length of the found offset string or 0 otherwise
+func parseSignedOffset(value string) int {
 	sign := value[0]
 	if sign != '-' && sign != '+' {
-		return 3
+		return 0
 	}
 	x, rem, err := leadingInt(value[1:])
 	if err != nil {
-		return 3
+		return 0
 	}
 	if sign == '-' {
 		x = -x
 	}
 	if x == 0 || x < -14 || 12 < x {
-		return 3
+		return 0
 	}
-	return 3 + len(value) - len(rem)
+	return len(value) - len(rem)
 }
 
 func parseNanoseconds(value string, nbytes int) (ns int, rangeErrString string, err error) {

src/time/format_test.go

@@ -427,6 +427,8 @@ var parseTimeZoneTests = []ParseTimeZoneTest{
 	{"ESASTT hi", 0, false}, // run of upper-case letters too long.
 	{"ESATY hi", 0, false},  // five letters must end in T.
 	{"WITA hi", 4, true},    // Issue #18251
+	{"+03 hi", 3, true},     // Issue #24071
+	{"-04 hi", 3, true},     // Issue #24071
 }
 
 func TestParseTimeZone(t *testing.T) {