RecyclerView with Dates

I want to filter in a recyclerView With Dates. To do this I make a query on the db SQLite, the dump in a cursor. I have created a method to which two variables type date di=fechaInicio and df=FechaFin are passed. My problem is that I do not know how to compare the dates and especially format them to be able to filter. This in my method code, I get an error at the time of formatting the date

private void consultarListaDietas(Date di, Date df) {
    SQLiteDatabase db = conn.getReadableDatabase();
    Dieta dieta = null;
    Date date = new Date();

    Cursor cursor = db.rawQuery("SELECT * FROM " + Utilidades.TABLA_DIETAS, null);
    if (di == null && df == null) {
        while (cursor.moveToNext()) {
            dieta = new Dieta();
            dieta.setId_Dieta(cursor.getInt(0));
            dieta.setNoches(cursor.getInt(1));
            dieta.setKm(cursor.getDouble(2));
            String fecha = cursor.getString(3);
            DateFormat iso8601Format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
            try {
                date = iso8601Format.parse(fecha);
            } catch (ParseException e) {
                Log.e(TAG, "Parsing ISO8601 datetime failed", e);
            }
            dieta.setFecha(date);
            dieta.setId_Usuario_FK(cursor.getInt(4));
            dieta.setId_Proyecto_FK(cursor.getInt(5));

        }

    } else {
        String fechaFin = df.toString();
        String fechaInicio = di.toString();
        cursor = null;
        String[] args = new String[]{fechaInicio, fechaFin};
        cursor = db.rawQuery("SELECT * FROM " + Utilidades.TABLA_DIETAS + " WHERE Fecha BETWEEN ? AND ?", args);
        while (cursor.moveToNext()) {
            dieta = new Dieta();
            dieta.setId_Dieta(cursor.getInt(0));
            dieta.setNoches(cursor.getInt(1));
            dieta.setKm(cursor.getDouble(2));
            String fecha = cursor.getString(3);
            DateFormat iso8601Format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
            try {
                date = iso8601Format.parse(fecha);
            } catch (ParseException e) {
                Log.e(TAG, "Parsing ISO8601 datetime failed", e);
            }
            dieta.setFecha(date);
            dieta.setId_Usuario_FK(cursor.getInt(4));
            dieta.setId_Proyecto_FK(cursor.getInt(5));

        }
    }
}

I pass the date selection code:

@Override

    public void onClick(View v) {

        //método para mostrar fecha cuando pinche el botón
        if (v==btinicio) {
            dia = calendario.get(Calendar.DAY_OF_MONTH);
            mes = calendario.get(Calendar.MONTH);
            ano = calendario.get(Calendar.YEAR);

            DatePickerDialog datePickerDialog = new DatePickerDialog(this, new DatePickerDialog.OnDateSetListener() {
                @Override
                public void onDateSet(DatePicker view, int year, int month, int dayOfMonth) {
                    txtinicio.setText(dayOfMonth + "/" + (month + 1) + "/" + year);
                }
            }, ano,mes,dia);
            datePickerDialog.show();
        }
        if (v == btfin) {
            dia = calendario.get(Calendar.DAY_OF_MONTH);
            mes = calendario.get(Calendar.MONTH);
            ano = calendario.get(Calendar.YEAR);

            DatePickerDialog datePickerDialog = new DatePickerDialog(this, new DatePickerDialog.OnDateSetListener() {
                @Override
                public void onDateSet(DatePicker view, int year, int month, int dayOfMonth) {
                    txtfin.setText(dayOfMonth + "/" + (month + 1) + "/" + year);
                }
            }, ano,mes,dia);
            datePickerDialog.show();
        }
    }