diff --git a/lib/mock_redis/string_methods.rb b/lib/mock_redis/string_methods.rb
index 56ba032f..e13cff6b 100644
--- a/lib/mock_redis/string_methods.rb
+++ b/lib/mock_redis/string_methods.rb
@@ -233,7 +233,7 @@ def set(key, value, options = {})
         if duration == 0
           raise Redis::CommandError, 'ERR invalid expire time in set'
         end
-        expire(key, duration / 1000.0)
+        pexpire(key, duration)
       end
 
       return_true ? true : 'OK'
diff --git a/spec/commands/set_spec.rb b/spec/commands/set_spec.rb
index 83189e87..4fc25cdc 100644
--- a/spec/commands/set_spec.rb
+++ b/spec/commands/set_spec.rb
@@ -59,9 +59,11 @@
 
       it 'accepts PX milliseconds' do
         key = 'mock-redis-test'
-        @mock.set(key, 1, px: 1000).should == 'OK'
+        @mock.set(key, 1, px: 500).should == 'OK'
         @mock.get(key).should_not be_nil
-        Time.stub(:now).and_return(@now + 2)
+        Time.stub(:now).and_return(@now + 300 / 1000.to_f)
+        @mock.get(key).should_not be_nil
+        Time.stub(:now).and_return(@now + 600 / 1000.to_f)
         @mock.get(key).should be_nil
       end
     end