Why is this date format used and where can it be used?

Question

                DateTime date1 = new DateTime(YY, MM, DD);
                DateTime date2 = new DateTime(YY, 4, 1);
                DateTime date3 = new DateTime(YY, 12, 31);
                DateTime date4 = new DateTime(YY, 3, 31);
                DateTime date5 = new DateTime(YY - 1, MM, DD);
                DateTime date6 = new DateTime(YY + 1, MM, DD);
                DateTime date7 = new DateTime(YY, 1, 1);