日期格式化 1.0

string a="2004.09.25 12:34:23";
   DateTime b=DateTime.Parse(a);
   string y=b.Year.ToString();
   string m=b.Month.ToString();
   string r=b.Day.ToString();
   if (y.Length<2)
   {
    y="0"+y;
   }
   if (m.Length<2)
   {
    m="0"+m;
   }
   string shu=y+"."+m+"."+r;             //2004.09.25
   string shu1=b.ToString("yyyy.mm.dd"); //2004.34.25
   string shu2=a.Substring(0,10);        //2004.09.25
   Response.Write(shu);