Not 100% sure what this will come up with. It will most likely be 01 dec 2017. It depends on how you want to use it. If you use it as a range then you will have to also find the end of the month
DateTime.ParseExact("1217", "MMYY", CultureInfo.InvariantCulture);
End of Month:
DateTime.ParseExact("1217", "MMYY", CultureInfo.InvariantCulture).AddMonths(1).AddDays(-1);