Define your own parse format string to use.
string formatString = "yyyyMMddHHmmss";
string sample = "20100611221912";
DateTime dt = DateTime.ParseExact(sample,formatString,null);
In case you got a datetime having milliseconds, use the following formatString
string format = "yyyyMMddHHmmssfff"
string dateTime = "20140123205803252";
DateTime.ParseExact(dateTime ,format,CultureInfo.InvariantCulture);
Thanks