A setTimeout replacement that pauses when browser window is not "visible"

setVisiTimeout.js

/**
 * This function uses the Page Visibility API to puase/restart the timeout such that
 * the time you specify equates to "visibility time".
 * Note: There is no clearTimeout capability - though it would not be hard to extend this to
 * 	provide cancel-ability
 *
 * Usage:
 * setVisiTimeout(function, ms, arg1, arg2, ... );
 *
 * LICENSE: Unlicense <http://unlicense.org/> / CC0
 */
function setVisiTimeout(fn,ms)
{
	function now() { return new Date().getTime(); }

	var started = now(); // record timestamp

	// Convert the arguments into an array, excluding the first
	// 2 arguments which are fn,ms - the remaining should be
	// passed on to the fn.
	var args = Array.prototype.slice.call(arguments,2);

	// visibilitychange event callback
	function vcEvent()
	{
		if(document.hidden) // clear timeout and sub out time passed
		{
			clearTimeout(to);
			ms -= now() - started;
		}
		else // restart timeout with time left
		{
			to = setTimeout(timeup,ms);
			started = now();
		}
	}

	// Full time has been used up - call user function with args
	function timeup()
	{
		if(document.removeEventListener)
			document.removeEventListener("visibilitychange", vcEvent);
		fn.apply(null, args);
	}

	// if not interrupted, this will trigger the timeup on its own
	var to = setTimeout(timeup, ms);

	// but if this event triggers, we'll have to do some bookkeeping
	if(document.addEventListener)
		document.addEventListener("visibilitychange", vcEvent);
}

Updated to support silly IE8.