JSFiddle - React, Tailwind, and code Playground

JavaScript

function strToDate(str) {
var year = parseInt(str.substring(0,4));
var month = parseInt(str.substring(4,6));
var day = parseInt(str.substring(6,8));
var hour = parseInt(str.substring(8,10));
var minute = parseInt(str.substring(10,12));
var second = parseInt(str.substring(12,14));

var date = new Date(year, month, day , hour, minute, second, 0);
return date.getTime();
}

function timeStampToDate(timestamp) {
 var d = new Date(timestamp);
    
    var diffStr = ""+(d.getFullYear()-1970)+":"+d.getMonth()+":"+(d.getDate()-1)+":"+(d.getHours()-1)+":"+d.getMinutes()+":"+d.getSeconds();
    return diffStr;
}

var lastExecution = "20150116165100"; //--- 16:51:00 16/01/2015
var currentTime =   "20150116170120"; //--- 17:01:20 16/01/2015
var lastExecutionDate = strToDate(lastExecution);
var currentTimeDate = strToDate(currentTime);

var difference = (currentTimeDate - lastExecutionDate);
var diffInTime = timeStampToDate(difference);
alert("Time difference: "+diffInTime);