diff --git a/lib/time.rb b/lib/time.rb
index 679cc82..c57d9ce 100644
--- a/lib/time.rb
+++ b/lib/time.rb
@@ -61,7 +61,9 @@ class << Time
     # Numeric time zones that include minutes, such as
     # <code>-10:00</code> or <code>+1330</code> will work, as will
     # simpler hour-only time zones like <code>-10</code> or
-    # <code>+13</code>.
+    # <code>+13</code>. These numeric time zones may also be prefixed
+    # with +GMT+ or +UTC+, such as <code>GMT+0900</code> or
+    # <code>UTC-05:30</code>.
     #
     # Textual time zones listed in ZoneOffset are also supported.
     #
@@ -83,10 +85,10 @@ class << Time
     def zone_offset(zone, year=nil)
       off = nil
       zone = zone.upcase
-      if /\A([+-])(\d\d)(:?)(\d\d)(?:\3(\d\d))?\z/ =~ zone
+      if /\A(?:GMT|UTC)?([+-])(\d\d)(:?)(\d\d)(?:\3(\d\d))?\z/ =~ zone
         off = ($1 == '-' ? -1 : 1) * (($2.to_i * 60 + $4.to_i) * 60 + $5.to_i)
-      elsif zone.match?(/\A[+-]\d\d\z/)
-        off = zone.to_i * 3600
+      elsif /\A(?:GMT|UTC)?([+-]\d\d)\z/ =~ zone
+        off = $1.to_i * 3600
       elsif ZoneOffset.include?(zone)
         off = ZoneOffset[zone] * 3600
       else
diff --git a/test/test_time.rb b/test/test_time.rb
index 4054bc4..e1ece64 100644
--- a/test/test_time.rb
+++ b/test/test_time.rb
@@ -266,6 +266,19 @@ def subtest_xmlschema_whitespaces(method)
     assert_equal(t, Time.__send__(method, s))
   end
 
+  def test_zone_offset
+    assert_equal(9 * 3600, Time.zone_offset("+09:00"))
+    assert_equal(9 * 3600, Time.zone_offset("+09"))
+    assert_equal(9 * 3600, Time.zone_offset("GMT+0900"))
+    assert_equal(9 * 3600, Time.zone_offset("gmt+09:00"))
+    assert_equal(9 * 3600, Time.zone_offset("UTC+09"))
+    assert_equal(-(5 * 3600 + 30 * 60), Time.zone_offset("GMT-05:30"))
+    assert_equal(-(5 * 3600 + 30 * 60), Time.zone_offset("UTC-0530"))
+    assert_equal(0, Time.zone_offset("GMT"))
+    assert_nil(Time.zone_offset("GMT+"))
+    assert_nil(Time.zone_offset("GMT+9:00"))
+  end
+
   def test_completion
     now = Time.local(2001,11,29,21,26,35)
     assert_equal(Time.local( 2001,11,29,21,12),
@@ -391,6 +404,17 @@ def test_parse_offset_hour_minute_second
     assert_equal(t, Time.parse("1200-02-15 BC 14:13:20-00:00:00"))
   end
 
+  def test_parse_gmt_utc_prefixed_offset
+    t = Time.utc(2026, 10, 10, 1, 47, 0)
+    # JavaScript's Date#toString format
+    assert_equal(t, Time.parse("Sat Oct 10 2026 10:47:00 GMT+0900 (Japan Standard Time)"))
+    assert_equal(9 * 3600, Time.parse("Sat Oct 10 2026 10:47:00 GMT+0900").utc_offset)
+    assert_equal(t, Time.parse("Fri Oct 09 2026 20:17:00 GMT-0530"))
+    assert_equal(t, Time.parse("2026-10-10 10:47:00 UTC+09:00"))
+    assert_equal(t, Time.parse("2026-10-10 10:47:00 UTC+09"))
+    assert_equal(t, Time.strptime("2026-10-10 10:47:00 GMT+0900", "%Y-%m-%d %H:%M:%S %Z"))
+  end
+
   def test_parse_custom_offset
     t = Time.at(-100000000000).utc
     assert_equal(t, Time.parse("1200-02-15 BC 14:13:20", zone: "UTC"))