Setting Expiry Date in Response.Cache

Hi,
how can i set an expiry date in Response.Cache from code behind.
below is my code, it work and appeared at no-cache at HTTP debugger Fiddler, but i can't set the expiry date.
please help me.

Response.Cache.SetCacheability(HttpCacheability.ServerAndNoCache);
                Response.Cache.SetAllowResponseInBrowserHistory(true);
                Response.Cache.SetExpires(Convert.ToDateTime("Mon,31 May 2010 00:00:00 GMT"));
                Response.Cache.SetValidUntilExpires(true);
thanks

...