When you change the system date format, it will apply to the entire date fields in the whole project. To change a specific field, you should use TryPaseExact method of DateTime .NET class as:
string pattern = "MM-dd-yy";
string dateValue = "30-12-2011";
DateTime parsedDate;
DateTime.TryParseExact(dateValue, pattern, null, DateTimeStyles.None, out parsedDate)