diff --git a/activesupport/lib/active_support/core_ext/date_time/conversions.rb b/activesupport/lib/active_support/core_ext/date_time/conversions.rb
index a4a6562c74..7c3a5eaace 100644
--- a/activesupport/lib/active_support/core_ext/date_time/conversions.rb
+++ b/activesupport/lib/active_support/core_ext/date_time/conversions.rb
@@ -80,8 +80,11 @@ class DateTime
 
   private
 
+  def offset_in_seconds
+    (offset * 86400).to_i
+  end
+
   def seconds_since_unix_epoch
-    seconds_per_day = 86_400
-    (self - ::DateTime.civil(1970)) * seconds_per_day
+    (jd - 2440588) * 86400 - offset_in_seconds + seconds_since_midnight
   end
 end
diff --git a/activesupport/test/core_ext/date_time_ext_test.rb b/activesupport/test/core_ext/date_time_ext_test.rb
index 183d58482d..21b7efdc73 100644
--- a/activesupport/test/core_ext/date_time_ext_test.rb
+++ b/activesupport/test/core_ext/date_time_ext_test.rb
@@ -427,6 +427,7 @@ class DateTimeExtCalculationsTest < ActiveSupport::TestCase
 
   def test_to_i
     assert_equal 946684800, DateTime.civil(2000).to_i
+    assert_equal 946684800, DateTime.civil(1999,12,31,19,0,0,Rational(-5,24)).to_i
   end
 
   protected