string str = "2012/3/31 14:34:22";

str = Convert.ToDateTime(str).ToString("yyyyMMdd"));

//str结果：20120331

------解决方案--------------------
DateTime.Now.ToString("yyyyMMddHHmmss")
HH大写是24小时制,小写是12小时制

------解决方案--------------------
		string s = "2012/3/31 14:34:22";
		Response.Write(DateTime.Parse(s).ToString("yyyyMMdd"));