ActionScript 3.0 :: Error #1009 When Subtracting Two Integers

I'm getting Error #1009 when subtracting two integers inside a class function.This code throws Error #1009:

ActionScript Code:
var difference:int = getTimer() - this.starttime;
if (difference >= 1) {

[code]....

starttime and countdown are declared as:

ActionScript Code:
public var countdown:int;
public var starttime:int;

and initialized in the class constructor by:

ActionScript Code:
countdown = Math.random()*60 + 90;
starttime = getTimer();