Verify that the StartDate column is a date, because if (in the dataset) it is a string-column, then you comparison will be a string comparison instead of a date comparison. That is probably why dates starting with "01" are never selected when you filter says " > 12/dd/yyyy".
As for the Select method, you have no choice and must convert to a string. Make sure to format the date correctly (I usually use yyyy/MM/dd format).