Countdown game timer help

Hi
I am looking for a countdown timer what will countdown mins +seconds
all i can find is a countdown to an event christmas day etc, but this wont work for what i need
i need a timer countdown that wont start again everytime a user enters the game i need a timer so all users enter the game at exactly the same time to the second, iv tried to do it using as3 php mysql but there is to much time difference between users how will i do this.
please help