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 41cf020f94..e93abfa4a3 100644
--- a/activesupport/lib/active_support/core_ext/date_time/calculations.rb
+++ b/activesupport/lib/active_support/core_ext/date_time/calculations.rb
@@ -1,4 +1,4 @@
-require 'rational'
+require 'rational' unless RUBY_VERSION >= '1.9.2'
 
 class DateTime
   class << self