diff --git a/ChangeLog b/ChangeLog
index e08c52a33b..efc522951e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sat Nov  9 14:50:09 2013  Zachary Scott  <e@zzak.io>
+
+	* lib/rss/rss.rb: [DOC] document Time#w3cdtf by @steveklabnik
+	  [Bug #8821] ありがとうかににく
+
 Sat Nov  9 14:29:04 2013  Zachary Scott  <e@zzak.io>
 
 	* ext/dl/cfunc.c: [DOC] fix typo in example [Bug #8944]
diff --git a/lib/rss/rss.rb b/lib/rss/rss.rb
index 55f7f9c805..6c89598e2d 100644
--- a/lib/rss/rss.rb
+++ b/lib/rss/rss.rb
@@ -3,6 +3,14 @@ require "time"
 class Time
   class << self
     unless respond_to?(:w3cdtf)
+      # This method converts a W3CDTF string date/time format to Time object.
+      #
+      # The W3CDTF format is defined here: http://www.w3.org/TR/NOTE-datetime
+      #
+      #   Time.w3cdtf('2003-02-15T13:50:05-05:00')
+      #   # => 2003-02-15 10:50:05 -0800
+      #   Time.w3cdtf('2003-02-15T13:50:05-05:00').class
+      #   # => Time
       def w3cdtf(date)
         if /\A\s*
             (-?\d+)-(\d\d)-(\d\d)
@@ -34,6 +42,13 @@ class Time
   end
 
   unless method_defined?(:w3cdtf)
+    # This method converts a Time object to a String. The String contains the
+    # time in W3CDTF date/time format.
+    #
+    # The W3CDTF format is defined here: http://www.w3.org/TR/NOTE-datetime
+    #
+    #  Time.now.w3cdtf
+    #  # => "2013-08-26T14:12:10.817124-07:00"
     def w3cdtf
       if usec.zero?
         fraction_digits = 0