How can i make an action performed every second?

I want to let this code:

var lefttime:int=1800;
function update () {
lefttime =lefttime-1;
}

Done every second.
How can i do this?

Thanks to everyone that answers (and sorry for my english)

var lefttime : int = 1800;

function Start() {
   //Pick either of these 2

   //Call DecreaseTime every 1 second
   InvokeRepeating( "DecreaseTime", 1 );

   //Call DecreaseTime every 1 second with a 3 second delay
   InvokeRepeating( "DecreaseTime", 3, 1 );
}

function DecreaseTime() {
   leftTime--;
}