JSFiddle - React, Tailwind, and code Playground

HTML

<select onChange="" name="stm_time[]">
   <option value="210">Ta je 210</option>
   <option value="511" selected="selected">Ta je 511</option>
   <option value="860">Ta je 860</option>
</select>

<select onChange="" name="stm_time[]">
   <option value="310">Ta je 310</option>
   <option value="912" selected="selected">Ta je 912</option>
   <option value="1260">Ta je 1260</option>
</select>

<p id="sum"> </p>

JavaScript

$(function() {    
    var sum = 0,
        sum_p = $('#sum');
        
    // jQuery each, ki gre čez value vseh selectov.
    $('select').each(function(i,v){
       sum += parseFloat($(v).val());   //jQeury val() funkcija dobi value selecta, parseFloat pa parsa v float da se lahko sesteva
    })
        
    sum_p.html(sum); 
    
    
    // jQuery change, da se dinamično spremeni sum    
    $('select').change(function(){
        
        sum = 0;
        $('select').each(function(i,v){
             sum += parseFloat($(v).val());
        })
            
        sum_p.html(sum);
            
    })    


});