decrement a value every seconed

This would be a task for a cron job.
MySQL has date/time functions, so you could write a trigger which checks and updates a time value on every operation (update,insert,delete), but it would not do anything without such an operation.
You could also code something like

select unix_timestamp('2012-01-01') - unix_timestamp(now()) as countdown;

ok then let me ask you another question,if 20,000 pepole load a page that refreshes itself every second, how affecting is it on server performance?
(a 5 kb page )

Ok, let me answer: the server load will be 20.000 pages per second. And with a 5 kb page it will be 100.000 kb per second.
Is this much or not? It all depends how the pages are constructed and delivered. Can you use a cache? Can you use the MySQL cache for repeated queries? And so on.
Apart from that, it seems to me like a silly idea to have a database pose as a countdown timer. Maybe you'd better tell us more about your problem, because this thread here gets somewhat misleading.

Ya this is irelivent now.
i came out really stupid.
i wanted to save a number of seconed left for a count down time and decrease it every seconed.
then i realised its a "silly idea" and just saved a timestamp and at the countdown i only compare the time at the moment with the time set in the timestamp and show the time left.
thanks for your help though!