addOnPositiveButtonClickListener 侦听器将所选日期作为 Long 值返回.
不要使用HeaderText.

The addOnPositiveButtonClickListener listener returns the selected date as Long value.
Don't use the HeaderText.

    start_DatePicker.addOnPositiveButtonClickListener(new MaterialPickerOnPositiveButtonClickListener<Long>() {
        @Override public void onPositiveButtonClick(Long selection) {
            Calendar calendar = Calendar.getInstance(TimeZone.getTimeZone("UTC"));
            calendar.setTimeInMillis(selection);
            SimpleDateFormat format = new SimpleDateFormat("yyyy MM dd");
            String formattedDate  = format.format(calendar.getTime());

        }
    });