Create a new string from the source string, with characters in wildcard  
positions replaced with some character (say, for example, '?').  Then in  
the string you provide to ParseExact(), use that replacement character in  
the same position where you replaced them in the source string.

For example:

    StringBuilder sb = new StringBuilder(strSource);

    sb.Remove(8, 4);
    sb.Insert(8, "????");

    return DateTime.ParseExact(sb.ToString(), "yyyyMMdd????HHmmss", null);

For all I know, there is some better, approved wildcard mechanism that I  
obviously don't know about.  But the above should work.  :)

Pete