Calculating age from unix timestamps in PHP

I couldn’t find a function already created for PHP that calculates someone’s age based on their birthdate and the current date (in unix timestamp format – seconds since 1970). First I tried this:

$age = date("Y",$endtime) - date("Y",$starttime);

But then I realized that this might not be completely accurate, base on the calendar, leap year, etc. So here’s what I ended up with:

function computeAge($starttime,$endtime)
{
	$age = date("Y",$endtime) - date("Y",$starttime);
	//if birthday didn't occur that last year, then decrement
	if(date("z",$endtime) < date("z",$starttime)) $age--;
	return $age;
}

Let me know if you have any improvements!