Common timer(countdown) for each end user

 Hi All,
Need a little help :). I need to have a timer countdown on the page which will count down and display the time (for ex. 00:00:30 then 00:00:29,   00:00:28 ... end etc.) this counter should be common and centralized for all users. In other words each user of the web site should see same timespan (for ex.   00:00:29) each second. so client side javascript counter will not help as I understand, because in that case all counters will not be centralized and will count by their own. 
So from the server side I tried to have a timer control with 1 second tick frequency, and on postback calculate timespan using DateTime.now object in the following way 
first I initialized the difference 
TimeSpan ts30sec = new TimeSpan(0, 0, 30); then calculated the initial value from where to start and stored it in cache
Cache["time"]=DateTime.Now.Add(ts30sec); < ...