What is the viewValue in the following AngularJS directive?

Question

I like to use the ui.bootstrap.datepicker directive with a specific format returned to ng-model. This has already been answered in this thread (DatePicker Have the Model come back as a specific format.). However, I don't understand what return +viewValue; does in the given solution. Can someone please tell me what this line of code mean? Thanks. Plunker here

app.directive('myformat', function() {
  return {
        require: 'ngModel',
        link: function(scope, element, attrs, ngModel) {
        ngModel.$parsers.push(function(viewValue) {
        return +viewValue; //????????? what is this doing?
            });
        }
    }
});