title: postAtTime not triggering
body: <p>I'm trying to make an application that changes the UI every minute, but I'm struggling a lot with the postAtTime function because I can't make it trigger. I've tried it with postDelay and it works, but postAtTime should give me more precission.</p>
  
  <p>I get the current time and set a calendar, then add a minute to the date and call the postAtTime, but it does not trigger. Let's see if you could help me.</p>
  
  <pre><code>   private Runnable showMinute = new Runnable() {
      public void run() {
          calendar = Calendar.getInstance();
          minute = calendar.get(Calendar.MINUTE);
          minute = minute % 10;
  
          switch (minute){
          case 0: numero.setImageResource(R.drawable.b0);
              break;
          case 1: numero.setImageResource(R.drawable.b1);
              break;
          case 2: numero.setImageResource(R.drawable.b2);
              break;
          case 3: numero.setImageResource(R.drawable.b3);
              break;
          case 4: numero.setImageResource(R.drawable.b4);
              break;
          case 5: numero.setImageResource(R.drawable.b5);
              break;
          case 6: numero.setImageResource(R.drawable.b6);
              break;
          case 7: numero.setImageResource(R.drawable.b7);
              break;
          case 8: numero.setImageResource(R.drawable.b8);
              break;
          case 9: numero.setImageResource(R.drawable.b9);
              break;
          default:
              break;
          }
  
  
          handler.removeCallbacks(mostrarHora);
          calendar.add(Calendar.MINUTE, 1);
          calendar.set(Calendar.SECOND, 0);
          calendar.set(Calendar.MILLISECOND, 0);
          handler.postAtTime(showMinute, calendar.getTimeInMillis());
      }
    };
  </code></pre>