time: accept anything between -23 and 23 as offset namezone name

time.Parse currently rejects numeric timezones names with UTC offsets
bigger than +12, but this is incorrect: there's a +13 timezone and a
+14 timezone:

  $ zdump Pacific/Kiritimati
  Pacific/Kiritimati  Mon Jun 25 02:15:03 2018 +14

For convenience, this cl changes the ranges of accepted offsets from
-14..+12 to -23..+23 (zero still excluded), i.e. every possible offset
that makes sense. We don't validate three-letter abbreviations for the
timezones names, so there's no need to be too strict on numeric names.

This change also fixes a bug in the parseTimeZone, that is currently
unconditionally returning true (i.e. valid timezone), without checking
the value returned by parseSignedOffset.

This fixes 5 of 17 time.Parse() failures listed in Issue #26032.

Updates #26032

Change-Id: I2f08ca9aa41ea4c6149ed35ed2dd8f23eeb42bff
Reviewed-on: https://go-review.googlesource.com/120558
Reviewed-by: Rob Pike <r@golang.org>

2 files changed, 17 insertions, 6 deletions

diff --git a/src/time/format.go b/src/time/format.go
index 237f28738b..f9cdbab3b8 100644
--- a/src/time/format.go
+++ b/src/time/format.go
@@ -1120,7 +1120,8 @@ func parseTimeZone(value string) (length int, ok bool) {
 	// Special Case 3: Some time zones are not named, but have +/-00 format
 	if value[0] == '+' || value[0] == '-' {
 		length = parseSignedOffset(value)
-		return length, true
+		ok := length > 0 // parseSignedOffset returns 0 in case of bad input
+		return length, ok
 	}
 	// How many upper-case letters are there? Need at least three, at most five.
 	var nUpper int
@@ -1152,7 +1153,7 @@ func parseTimeZone(value string) (length int, ok bool) {
 
 // parseGMT parses a GMT time zone. The input string is known to start "GMT".
 // The function checks whether that is followed by a sign and a number in the
-// range -14 through 12 excluding zero.
+// range -23 through +23 excluding zero.
 func parseGMT(value string) int {
 	value = value[3:]
 	if len(value) == 0 {
@@ -1163,7 +1164,7 @@ func parseGMT(value string) int {
 }
 
 // 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.
+// The function checks for a signed number in the range -23 through +23 excluding zero.
 // Returns length of the found offset string or 0 otherwise
 func parseSignedOffset(value string) int {
 	sign := value[0]
@@ -1177,7 +1178,7 @@ func parseSignedOffset(value string) int {
 	if sign == '-' {
 		x = -x
 	}
-	if x == 0 || x < -14 || 12 < x {
+	if x == 0 || x < -23 || 23 < x {
 		return 0
 	}
 	return len(value) - len(rem)
diff --git a/src/time/format_test.go b/src/time/format_test.go
index 68a4d3ddb0..c3552f4161 100644
--- a/src/time/format_test.go
+++ b/src/time/format_test.go
@@ -427,8 +427,18 @@ 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
+	// Issue #24071
+	{"+03 hi", 3, true},
+	{"-04 hi", 3, true},
+	// Issue #26032
+	{"-11", 3, true},
+	{"-12", 3, true},
+	{"-23", 3, true},
+	{"-24", 0, false},
+	{"+13", 3, true},
+	{"+14", 3, true},
+	{"+23", 3, true},
+	{"+24", 0, false},
 }
 
 func TestParseTimeZone(t *testing.T) {