Getting last months data from database

Hi there,

I am trying to work out how I can get the previous months results from my database.  This is what I have so far but I just cannot figure out what should be in the where line.

PHP Code:

public function get_last_month(){
        $this->db->select('*');
        $this->db->from('test_table');
        $this->db->where('date', '');
        $query = $this->db->get();
        return $query->result();
    } 


Can anyone help please?

Thanks,

Doomie