• HTML
• CSS
• JS

HTML

<div class="time-elapsed" id="timeelapsed"></div>

CSS

.time-elapsed{
    font-family:arial;
    background:#e9ecef;
    text-align:center;
    margin:0.5rem 0rem;
    padding:2rem 0rem;
    font-size:6rem;
    font-weight:bold;
    color:#633974;
}

JS

var date1 = new Date();

function time(){

    var date2  = new Date();
    var rest  = date2.getTime() - date1.getTime();

    var r1=rest%(1000*60*60);//min
    var r2=r1%(1000*60);//segund
    var r3=r2%(1000)//ms
    
    var hour   = rest/(1000*60*60);
    var min    = r1/(1000*60);
    var segund = r2/(1000);

    hour   = Math.trunc(hour);
    min    = Math.trunc(min);
    segund = Math.trunc(segund);
    
    if(hour<10){
        hour="0"+hour;
    }
    if(min<10){ 
        min = "0"+min;
    }
    if(segund<10){
        segund="0"+segund;
    }
    
    var tiempo = hour + ":" + min + ":" + segund;
    var elem = document.getElementById("timeelapsed");
    
    elem.innerHTML=tiempo;

}

setInterval("time()",1000);