watch expression new value always equals to old value - angularjs

Question

i'm using two AngularJS Bootstrap-ui Datepicker directives in a single controller.
The code is very simple, the view :
<div class="row">
<div class="col-md-6">
<p class="input-group">
<input type="text" class="form-control" datepicker-popup="dd-MM-yyyy" ng-model="startDate" is-open="startDateOpened" min="minDate" max="'2015-06-22'" ng-required="true" close-text="Close" />
<span class="input-group-btn">
<button class="btn btn-default" ng-click="open($event, 'startDateOpened')"><i class="glyphicon glyphicon-calendar"></i></button>
</span>
</p>
</div>
<div class="col-md-6">
<p class="input-group">
<input type="text" class="form-control" datepicker-popup="dd-MM-yyyy" ng-model="endDate" is-open="endDateOpened" min="minDate" max="'2015-06-22'" ng-required="true" close-text="Close" />
<span class="input-group-btn">
<button class="btn btn-default" ng-click="open($event, 'endDateOpened')"><i class="glyphicon glyphicon-calendar"></i></button>
</span>
</p>
</div>
</div>
The controller (part of it).
var now = new Date();
var startDate = $scope.startDate = new Date(now.getFullYear(), now.getMonth(), 1); // beginning of month
var endDate = $scope.endDate = now;
$scope.$watchCollection('[startDate, endDate]', function (newVals, oldVals) {
if(!(newVals[0] === oldVals[0] && newVals[1] === oldVals[1])){
console.log('old values', oldVals);
console.log('new values', newVals);
print();
}
});
function print() {
console.log('startDate !', startDate);
console.log('endDate !', endDate);
}
$scope.open = function($event, name) {
$event.preventDefault();
$event.stopPropagation();
$scope[name] = true;
};
I am assigning two models - startDate and endDate. Everything looks like it is working fine, when I choose a date in either of the date pickers $scope.$watchCollection() callback is fired up but oldVals is always eqaul to newVals so the model doesn't really change (even though the change event listener is fired).
What am I doing wrong ? I'm guessing it's a lack of understanding something basic :)
Thanks
EDIT
#Maxim's answer of using deep watch did solve my problem of not going into the if statement. Now my other problem is that print function inside the callback always prints the old values of startDate and endDate

Answer 1

Try $watch with flag true aka deep watch:
$scope.$watch('[startDate, endDate]', function (newVals, oldVals) {
if(!(newVals[0] === oldVals[0] && newVals[1] === oldVals[1])){
console.log('old values', oldVals);
console.log('new values', newVals);
startDate = newVals[0];
endDate = newVals[1];
}
}, true);
Plunker