You should have to use DateTime.TryParseExact.
var newDate = DateTime.ParseExact("20111120",
"yyyyMMdd",
CultureInfo.InvariantCulture);
OR
string str = "20111021";
string[] format = {"yyyyMMdd"};
DateTime date;
if (DateTime.TryParseExact(str,
format,
System.Globalization.CultureInfo.InvariantCulture,
System.Globalization.DateTimeStyles.None,
out date))
{
//valid
}