bt2.setOnClickListener(new View.OnClickListener(){
    @Override
    public void onClick(View v){
        if (cd!=null)cd=null;
        cd =Calendar.getInstance();
        DatePickerDialog datePickerDialog = new DatePickerDialog(MainActivity.this, new DatePickerDialog.OnDateSetListener() {
            @Override
 
            public void onDateSet(DatePicker view, int year, int month, int dayOfMonth) {
                cd.set(Calendar.YEAR,year);
                cd.set(Calendar.MONTH,month);
                cd.set(Calendar.DAY_OF_MONTH,dayOfMonth);
            }

        }, cd.get(Calendar.YEAR), cd.get(Calendar.MONTH), cd.get(Calendar.DAY_OF_MONTH));
 
        datePickerDialog.show();
    }
});

安卓原生的日期選擇器