Android : CountDownTimer not accurate

I have an Sndroid service running in the background. I want to be notified after a specific period of time (22 seconds), so I write:

private CountDownTimer mCountDownTimer = new CountDownTimer(22*1000,22*1000) {

     public void onTick(long millisUntilFinished) {}

     public void onFinish()
     {
         doSomething();
     }
  };

I run this, and get the notification after 40 seconds, and even 50 seconds. Am I doing something wrong? How can this be done?