convert string to datetime ติด error String was not recognized as a valid DateTime.

newDateOfLetter = "2014-Jul-09";
int dateOfLetterMonth = Convert.ToDateTime(newDateOfLetter).Month;
string newPeriodId = (dateOfLetterMonth <= 6) ? "1" : "2";

txtDateOfLetter = "2014-Jul-09";
int dateOfLetterMonth = Convert.ToDateTime(txtDateOfLetter).Month;
string txtPeriodId = (dateOfLetterMonth <= 6) ? "1" : "2";

txtDateOfLetter = "2014-Jul-09";
DateTime dateOfLetterMonth = DateTime.ParseExact(txtDateOfLetter, "yyyy-MMM-dd", CultureInfo.InvariantCulture);
string txtPeriodId = (dateOfLetterMonth.Month <= 6) ? "1" : "2";