How to create a JQuery Clock / Timer

How about the best of both worlds? I combined the answer with the OP's format.

function pretty_time_string(num) {
    return ( num < 10 ? "0" : "" ) + num;
  }

var start = new Date;    

setInterval(function() {
  var total_seconds = (new Date - start) / 1000;   

  var hours = Math.floor(total_seconds / 3600);
  total_seconds = total_seconds % 3600;

  var minutes = Math.floor(total_seconds / 60);
  total_seconds = total_seconds % 60;

  var seconds = Math.floor(total_seconds);

  hours = pretty_time_string(hours);
  minutes = pretty_time_string(minutes);
  seconds = pretty_time_string(seconds);

  var currentTimeString = hours + ":" + minutes + ":" + seconds;

  $('.timer').text(currentTimeString);
}, 1000);

You're looking for the setInterval function, which runs a function every x milliseconds.

For example:

var start = new Date;

setInterval(function() {
    $('.Timer').text((new Date - start) / 1000 + " Seconds");
}, 1000);

################## JQuery (use API) #################   
 $(document).ready(function(){
         function getdate(){
                var today = new Date();
            var h = today.getHours();
            var m = today.getMinutes();
            var s = today.getSeconds();
             if(s<10){
                 s = "0"+s;
             }
             if (m < 10) {
                m = "0" + m;
            }
            $("h1").text(h+" : "+m+" : "+s);
             setTimeout(function(){getdate()}, 500);
            }

        $("button").click(getdate);
    });

################## HTML ###################
<button>start clock</button>
<h1></h1>

setInterval as suggested by SLaks was exactly what I needed to make my timer. (Thanks mate!)

Using setInterval and this great blog post I ended up creating the following function to display a timer inside my "box_header" div. I hope this helps anyone else with similar requirements!

 function get_elapsed_time_string(total_seconds) {
  function pretty_time_string(num) {
    return ( num < 10 ? "0" : "" ) + num;
  }

  var hours = Math.floor(total_seconds / 3600);
  total_seconds = total_seconds % 3600;

  var minutes = Math.floor(total_seconds / 60);
  total_seconds = total_seconds % 60;

  var seconds = Math.floor(total_seconds);

  // Pad the minutes and seconds with leading zeros, if required
  hours = pretty_time_string(hours);
  minutes = pretty_time_string(minutes);
  seconds = pretty_time_string(seconds);

  // Compose the string for display
  var currentTimeString = hours + ":" + minutes + ":" + seconds;

  return currentTimeString;
}

var elapsed_seconds = 0;
setInterval(function() {
  elapsed_seconds = elapsed_seconds + 1;
  $('#box_header').text(get_elapsed_time_string(elapsed_seconds));
}, 1000);