anchor.diff:        });
   
     _advanceTo(DateTime to) {
  -    if(to.millisecondsSinceEpoch > _now.millisecondsSinceEpoch) {
  +    if (to.millisecondsSinceEpoch > _now.millisecondsSinceEpoch) {
         _now = to;
       }
     }
  
positive.code: 
    _scheduleTimer(Zone self, ZoneDelegate parent, Zone zone) {
  
      if(!_waitingForTimer && _advancingTo != null) {
        var next = _getNextTimer();
        var completeTimer = next != null ?
            self.bindCallback(() => _runTimer(next), runGuarded: true) :
  
negative.code:   DateTime now() => _now;
  
    Future advance(Duration duration) {
      if (duration.inMicroseconds < 0) {
        return new Future.error(
            new ArgumentError('Cannot call advance with negative duration'));
      }
      if (_advancingTo != null) {
        return new Future.error(
            new StateError('Cannot advance until previous advance is complete.'));
      }
  