обновил ваш код следующим кодом,
DatePickerDialog dpd = null;
protected Dialog onCreateDialog(int id) {
switch (id) {
case DATE_DIALOG_ID:
Calendar c = Calendar.getInstance();
mYear = c.get(c.YEAR);
mMonth = c.get(c.MONTH);
mDay = c.get(c.DAY_OF_MONTH);
dpd = new DatePickerDialog(this, mDateSetListener, mYear, mMonth, mDay);
break; // you forgot this line in your code
default:
return null;
}
}