diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 23ca8246e260039e91fedc90ac3ab7f168f43d0b..1056b6696fee0972fc930b2b933fb0dc345118a8 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Add `Time#sec_fraction` to return the fraction of a second as a `Rational`.
+
+    *Andrew White*
+
 *   Add `ActiveSupport.to_time_preserves_timezone` config option to control
     how `to_time` handles timezones. In Ruby 2.4+ the behavior will change
     from converting to the local system timezone to preserving the timezone
diff --git a/activesupport/lib/active_support/core_ext/time/calculations.rb b/activesupport/lib/active_support/core_ext/time/calculations.rb
index 768c9a1b2cc8148a7694c40e059a9cd5b6df41b8..9c019e10b5fef9c6b90368608878d67e5294890f 100644
--- a/activesupport/lib/active_support/core_ext/time/calculations.rb
+++ b/activesupport/lib/active_support/core_ext/time/calculations.rb
@@ -73,6 +73,13 @@ def seconds_until_end_of_day
     end_of_day.to_i - to_i
   end
 
+  # Returns the fraction of a second as a +Rational+
+  #
+  #    Time.new(2012, 8, 29, 0, 0, 0.5).sec_fraction # => (1/2)
+  def sec_fraction
+    Rational(nsec, 1000000000)
+  end
+
   # Returns a new Time where one or more of the elements have been changed according
   # to the +options+ parameter. The time options (<tt>:hour</tt>, <tt>:min</tt>,
   # <tt>:sec</tt>, <tt>:usec</tt>, <tt>:nsec</tt>) reset cascadingly, so if only