diff --git a/ChangeLog b/ChangeLog
index 0d1923ca9b..c125a4100a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Mon Sep  9 09:14:58 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* tool/rbinstall.rb (Gem::Specification#initialize): default date to
+	  RUBY_RELEASE_DATE.  [ruby-core:57072] [Bug #8878]
+
+	* tool/rbinstall.rb (Gem::Specification#to_ruby): add date.
+
 Sun Sep  8 16:01:54 2013  Tanaka Akira  <akr@fsij.org>
 
 	* rational.c (f_gcd): Relax the condition to use GMP.
diff --git a/tool/rbinstall.rb b/tool/rbinstall.rb
index 29866dfffd..b47b6e15fe 100755
--- a/tool/rbinstall.rb
+++ b/tool/rbinstall.rb
@@ -560,6 +560,7 @@ module Gem
       super
       yield(self) if defined?(yield)
       self.executables ||= []
+      self.date ||= RUBY_RELEASE_DATE
     end
 
     def self.load(path)
@@ -573,6 +574,7 @@ module Gem
 Gem::Specification.new do |s|
   s.name = #{name.dump}
   s.version = #{version.dump}
+  s.date = #{date.dump}
   s.summary = #{summary.dump}
   s.description = #{description.dump}
   s.homepage = #{homepage.dump}