diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 2e465a0310..2f5ed35f46 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,12 @@
+*   Fix duration being rounded to a full second.
+    ```
+      time = DateTime.parse("2018-1-1")
+      time += 0.51.seconds
+    ```
+    Will now correctly add 0.51 second and not 1 full second.
+
+    *Edouard Chin*
+
 *   Deprecate `ActiveSupport::Multibyte::Unicode#normalize` and `ActiveSuppport::Multibyte::Chars#normalize`
     in favor of `String#unicode_normalize`
 
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 e61b23f842..bc670c3e76 100644
--- a/activesupport/lib/active_support/core_ext/date_time/calculations.rb
+++ b/activesupport/lib/active_support/core_ext/date_time/calculations.rb
@@ -110,7 +110,7 @@ class DateTime
   # instance time. Do not use this method in combination with x.months, use
   # months_since instead!
   def since(seconds)
-    self + Rational(seconds.round, 86400)
+    self + Rational(seconds, 86400)
   end
   alias :in :since
 
diff --git a/activesupport/test/core_ext/date_time_ext_test.rb b/activesupport/test/core_ext/date_time_ext_test.rb
index 894fb80cba..f9f6b21c9b 100644
--- a/activesupport/test/core_ext/date_time_ext_test.rb
+++ b/activesupport/test/core_ext/date_time_ext_test.rb
@@ -152,8 +152,8 @@ class DateTimeExtCalculationsTest < ActiveSupport::TestCase
     assert_equal DateTime.civil(2005, 2, 22, 11, 10, 10), DateTime.civil(2005, 2, 22, 10, 10, 10).since(3600)
     assert_equal DateTime.civil(2005, 2, 24, 10, 10, 10), DateTime.civil(2005, 2, 22, 10, 10, 10).since(86400 * 2)
     assert_equal DateTime.civil(2005, 2, 24, 11, 10, 35), DateTime.civil(2005, 2, 22, 10, 10, 10).since(86400 * 2 + 3600 + 25)
-    assert_equal DateTime.civil(2005, 2, 22, 10, 10, 11), DateTime.civil(2005, 2, 22, 10, 10, 10).since(1.333)
-    assert_equal DateTime.civil(2005, 2, 22, 10, 10, 12), DateTime.civil(2005, 2, 22, 10, 10, 10).since(1.667)
+    assert_not_equal DateTime.civil(2005, 2, 22, 10, 10, 11), DateTime.civil(2005, 2, 22, 10, 10, 10).since(1.333)
+    assert_not_equal DateTime.civil(2005, 2, 22, 10, 10, 12), DateTime.civil(2005, 2, 22, 10, 10, 10).since(1.667)
   end
 
   def test_change