diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 2515d378fd..b07ab749a3 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,5 +1,10 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Prevent `DateTime#change` from truncating the second fraction, when seconds
+    do not need to be changed.
+
+    *Chris Baynes*
+
 *   Added `ActiveSupport::TimeWithZone#to_r` for `Time#at` compatibility.
 
     Before this change:
diff --git a/activesupport/lib/active_support/core_ext/date_time/calculations.rb b/activesupport/lib/active_support/core_ext/date_time/calculations.rb
index 97aad008f5..9f0864d9bb 100644
--- a/activesupport/lib/active_support/core_ext/date_time/calculations.rb
+++ b/activesupport/lib/active_support/core_ext/date_time/calculations.rb
@@ -59,7 +59,7 @@ class DateTime
       options.fetch(:day, day),
       options.fetch(:hour, hour),
       options.fetch(:min, options[:hour] ? 0 : min),
-      options.fetch(:sec, (options[:hour] || options[:min]) ? 0 : sec),
+      options.fetch(:sec, (options[:hour] || options[:min]) ? 0 : sec + sec_fraction),
       options.fetch(:offset, offset),
       options.fetch(:start, start)
     )
diff --git a/activesupport/test/core_ext/date_time_ext_test.rb b/activesupport/test/core_ext/date_time_ext_test.rb
index 59ca2a4059..7be578599b 100644
--- a/activesupport/test/core_ext/date_time_ext_test.rb
+++ b/activesupport/test/core_ext/date_time_ext_test.rb
@@ -129,6 +129,9 @@ class DateTimeExtCalculationsTest < ActiveSupport::TestCase
     assert_equal DateTime.civil(2005,2,22,16),       DateTime.civil(2005,2,22,15,15,10).change(:hour => 16)
     assert_equal DateTime.civil(2005,2,22,16,45),    DateTime.civil(2005,2,22,15,15,10).change(:hour => 16, :min => 45)
     assert_equal DateTime.civil(2005,2,22,15,45),    DateTime.civil(2005,2,22,15,15,10).change(:min => 45)
+
+    # datetime with fractions of a second
+    assert_equal DateTime.civil(2005,2,1,15,15,10.7), DateTime.civil(2005,2,22,15,15,10.7).change(:day => 1)
   end
 
   def test_advance