diff --git a/lib/aws/s3/authentication.rb b/lib/aws/s3/authentication.rb
index 0efbc7a..2a18acf 100644
--- a/lib/aws/s3/authentication.rb
+++ b/lib/aws/s3/authentication.rb
@@ -115,7 +115,7 @@ def initialize(*args)
           #   3) The current time in seconds since the epoch plus the default number of seconds (60 seconds)
           def expires
             return options[:expires] if options[:expires]
-            date.to_i + expires_in
+            @expires ||= date.to_i + expires_in
           end
           
           def expires_in
@@ -218,4 +218,4 @@ def only_path
       end
     end
   end
-end
\ No newline at end of file
+end