diff --git a/ChangeLog b/ChangeLog
index b268376359..a4f214f34a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Sep 30 03:44:36 2015  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* lib/rss/rss.rb (Time#w3cdtf): fix zero-trimmed width of fraction
+	  digits.  [ruby-core:70667] [Bug #11509]
+
 Wed Sep 30 03:34:25 2015  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* re.c (rb_memsearch_wchar, rb_memsearch_qchar): test matching
diff --git a/lib/rss/rss.rb b/lib/rss/rss.rb
index 4f6732b4d2..dbea3b5344 100644
--- a/lib/rss/rss.rb
+++ b/lib/rss/rss.rb
@@ -53,7 +53,7 @@ class Time
       if usec.zero?
         fraction_digits = 0
       else
-        fraction_digits = Math.log10(usec.to_s.sub(/0*$/, '').to_i).floor + 1
+        fraction_digits = strftime('%6N').index(/0*\z/)
       end
       xmlschema(fraction_digits)
     end
diff --git a/test/rss/test_to_s.rb b/test/rss/test_to_s.rb
index 4ef83ce18b..d97cf9d4b0 100644
--- a/test/rss/test_to_s.rb
+++ b/test/rss/test_to_s.rb
@@ -102,6 +102,12 @@ module RSS
       assert_textinput20(@textinput_info, rss.textinput)
     end
 
+    def test_time_w3cdtf
+      assert_equal("2015-09-05T01:25:48.0001Z",
+                   Time.utc(2015, 9, 5, 1, 25, 48, 100).w3cdtf,
+                   '[ruby-core:70667] [Bug #11509]')
+    end
+
     private
     def setup_xml_declaration_info
       @version = "1.0"
diff --git a/version.h b/version.h
index a5e0c86ca6..5714dd90bf 100644
--- a/version.h
+++ b/version.h
@@ -1,6 +1,6 @@
 #define RUBY_VERSION "2.2.4"
 #define RUBY_RELEASE_DATE "2015-09-30"
-#define RUBY_PATCHLEVEL 177
+#define RUBY_PATCHLEVEL 178
 
 #define RUBY_RELEASE_YEAR 2015
 #define RUBY_RELEASE_MONTH 9