diff --git a/CHANGES b/CHANGES
index 61404921..06833872 100644
--- a/CHANGES
+++ b/CHANGES
@@ -18,6 +18,9 @@
  * Added `request.accept?` and `request.preferred_type` to ease dealing with
    `Accept` headers. (Konstantin Haase)
 
+ * Helpers dealing with time, like `expires`, handle objects that pretend to be
+   numbers, lik `ActiveSupport::Duration`, better. (Konstantin Haase)
+
 = 1.2.2 / 2011-04-08
 
  * The `:provides => :js` condition now matches both `application/javascript`
diff --git a/lib/sinatra/base.rb b/lib/sinatra/base.rb
index 08e528b3..2a335fb3 100644
--- a/lib/sinatra/base.rb
+++ b/lib/sinatra/base.rb
@@ -340,7 +340,7 @@ module Sinatra
     def expires(amount, *values)
       values << {} unless values.last.kind_of?(Hash)
 
-      if Integer === amount
+      if amount.is_a? Integer
         time    = Time.now + amount
         max_age = amount
       else
@@ -405,7 +405,7 @@ module Sinatra
     def time_for(value)
       if value.respond_to? :to_time
         value.to_time
-      elsif Time === value
+      elsif value.is_a? Time
         value
       elsif value.respond_to? :new_offset
         # DateTime#to_time does the same on 1.9
@@ -415,7 +415,7 @@ module Sinatra
       elsif value.respond_to? :mday
         # Date#to_time does the same on 1.9
         Time.local(value.year, value.mon, value.mday)
-      elsif Numeric === value
+      elsif value.is_a? Numeric
         Time.at value
       else
         Time.parse value.to_s
diff --git a/test/helpers_test.rb b/test/helpers_test.rb
index f20bf2ce..d8b791ad 100644
--- a/test/helpers_test.rb
+++ b/test/helpers_test.rb
@@ -638,6 +638,14 @@ class HelpersTest < Test::Unit::TestCase
         get '/baz' do
           expires Time.at(0)
         end
+
+        get '/blah' do
+          obj = Object.new
+          def obj.method_missing(*a, &b) 60.send(*a, &b) end
+          def obj.is_a?(thing) 60.is_a?(thing) end
+          expires obj, :public, :no_cache
+          'Hello World'
+        end
       end
     end
 
@@ -660,6 +668,11 @@ class HelpersTest < Test::Unit::TestCase
       get '/baz'
       assert_equal 'Thu, 01 Jan 1970 00:00:00 GMT', response['Expires']
     end
+
+    it 'accepts values pretending to be a Numeric (like ActiveSupport::Duration)' do
+      get '/blah'
+      assert_equal ['public', 'no-cache', 'max-age=60'], response['Cache-Control'].split(', ')
+    end
   end
 
   describe 'last_modified' do