error Undefined offset, how is fix it?

[eluser]SaSa[/eluser]
what is this error?

Code:

A PHP Error was encountered
    Severity: Notice    
    Message: Undefined offset: 2  
    Filename: admin/tour.php    
    Line Number: 59
    
    A PHP Error was encountered    
    Severity: Notice    
    Message: Undefined offset: 1    
    Filename: admin/tour.php    
    Line Number: 59

    0

my codes:

Code:

function autocomplete_datego(){
            if($this->session->userdata('logged_in')) {
            $date_go = $this->input->post('date_go');
            $jdate = jgmdate("Y/m/j");
            list($year_now, $month_now, $day_now) = explode('/', $jdate, 3);

    //////////////////////////////// line 59 ///////////////
            list($year, $month, $day) = explode('/', $date_go, 3); // line 59
    //////////////////////////////// end line 59 ///////////////

            if($year>=$year_now && $month<=12 ) {
                $j2g = $this->convert_date->JalaliToGregorian($year, $month, $day);
                echo $j2g[0]."/".$j2g[1]."/".$j2g[2];
            }else {
                echo '0';
            }
            }
        }