Adding one month to selected date

[eluser]Unknown[/eluser]
I have date picker, where user can pick any date and it is update as start date. There is also end date, one month from selected date. How to write function?

Code:

function clanarina()
    {
        //date is in format: 02/08/2011 (day, month, year), so I need to explode to get mysql date
        $selected_date = explode('/', $this->input->post('datepicker'));
        $dan = $selected_date ['0'];
        $mesec = $selected_date ['1'];
        $godina = $selected_date ['2'];
        $selected_date = $godina . "-" . $mesec . "-" . $dan;
        //here i tried to add one month, but in db all I get is 0000-00-00
        $expire =  strtotime(date("Y-m-d", strtotime($selected_date ))) . " +1 month";
        $data = array(
               'datum_pocetak' => $selected_date ,
                'datum_istek' => $expire
            );
$this->db->where('clan_id', $this->uri->segment(4));
$this->db->update('clanarina', $data);
$this->clan();
    }