This is really simple to do. Depends on the timer you want to use though. Are you asking for graphical representation of the time, or just something that executes a function after X seconds?
AS2:
ActionScript Code:
function toExecute()
{}
setInterval( 1000 /*This is any number of milliseconds, 1000 in one second.*/, toExecute /*The function to execute after the number of milliseconds.*/);
AS3:
ActionScript Code:
var timer:Timer = new Timer(1000);
timer.addEventListener(TimerEvent.TIMER, functionToExecute);
function functionToExecute(e:Event)
{}
//Same idea applies here, 1000 is in milliseconds, and if you are using AS3 you should know AS3 well enough to get this code.
All I can tell you is this: Do NOT buy some sort of device to keep time unless it is exactly what you wanted and you can't get help. You can get your help here for free, and actually I'll do it for you if you want to once I get to my home computer. (At school right now.)
Good luck.