I have just finished in javascript, but the code seems a little long.
html:
<div id="process"></div>
javascript:
var process = document.getElementById('process');
var cur = '15:25:00';
var end = '16:25:00';
cur = cur.split(':');
end = end.split(':');
var x = parseInt(cur[0], 10)*3600+parseInt(cur[1], 10)*60+parseInt(cur[2], 10);
var y = parseInt(end[0], 10)*3600+parseInt(end[1], 10)*60+parseInt(end[2], 10);
showTime(y);
var timer = setInterval(function(){
y--;
showTime(y);
if(y<=x){
clearInterval(timer);
}
}, 1000);
function showTime(diff){
var hour = parseInt(diff/3600, 10);
var minute = parseInt((diff - hour*3600)/60, 10);
var second = diff - hour*3600-minute*60;
hour = hour<10 ? '0'+hour : hour;
minute = minute<10 ? '0'+minute : minute;
second = second<10 ? '0'+second : second;
var time = hour+':'+minute+':'+second;
process.innerHTML = time;
}