Select SLA Plan for calculate Due Date in osticket program

Question

Today I'am have a problem modify code for Program "osTicket (v1.9.12)" for calculate Due Date

I think post ask in forum : http://osticket.com/forum/discussions .. I don't know ... how to use post ask here ? ... where ?

Request : value $row['grace_period'] calculator date for Due Date

Action :

1. on select change SLA Plan
2. auto active immediately [client site]
3. conditions check if (id) [server site]
4. query $row['grace_period']
5. auto calculate Due Date and Time
6. display resulte date and time in text Box [Due Date]

Example :

1. select : SLA Plan :Default SLA (48 hours - Active)
2. Today : 2015/10/12 14:00:10
3. resulte [Due Date] : 2015/10/14 14:00:10

class.sla.php

function getSLAs() {

    $slas=array();

    $sql='SELECT id, name, isactive, grace_period FROM '.SLA_TABLE.' ORDER BY name';
    if(($res=db_query($sql)) && db_num_rows($res)) {
        while($row=db_fetch_array($res))
            $slas[$row['id']] = sprintf(__('%s (%d hours - %s) '
                    /* Tokens are <name> (<#> hours - <Active|Disabled>) */),
                    $row['name'],
                    $row['grace_period'],
                    $row['isactive']?__('Active'):__('Disabled'));
    }

    return $slas;
}

ticket-open.php

<td width="160">
            <?php echo __('SLA Plan');?>:
        </td>
        <td>            
           <select id="slaId" name="slaId" onchange="choose_sla()">
               <option value="0" selected="selected" >&mdash; <?php echo __('System Default');?> &mdash;</option>
                <?php
       if($slas=SLA::getSLAs()) {
             foreach($slas as $id =>$name) {                
              echo sprintf('<option value="%d" %s>%s</option>',
                 $id, ($info['slaId']==$id)?'selected="selected"':'',$name);        
    }   
        }
                ?>
            </select>

            &nbsp;<font class="error">&nbsp;<?php echo $errors['slaId']; ?></font>
        </td> 

//test selected  
<p id="cal_due"></p>

<script>
function choose_sla() {

    var x = document.getElementById("slaId").value;

    document.getElementById("cal_due").innerHTML = "You selected: " + x;    
    }

Check out put :

print_r($slas);
//Array ( [1] => Default SLA (48 hours - Active)
//[6] => SLA 93.33% ( 2880 นาที - 48 ชม.) (48 hours - Active)
//[5] => SLA 99.10% (388.8 นาที - 6.29 ชม.) (6 hours - Active)
//[3] => SLA 99.40% (129.6 นาที - 5 ชม.) (5 hours - Active)
//[2] => SLA 99.80% (86.4 นาที - 1.26 ชม.) (1 hours - Active) )

Result Current :

You selected : id = 1 {6,5,3,2} <<< value on select change

I can't query and return value of $row['grace_period'] = {48,6.29,5,1.26} ...

What you have a suggestion for my ?

Thank you very much ...