Edit in JSFiddle

var app = angular.module('app', ['wj']);
app.controller('appCtrl', function ($scope) {
    $scope.btn_click = function (e, args) {
        $scope.date = null;
        $scope.month = new Date(2017, 0, 1);
    }
});

<div ng-app="app" ng-controller="appCtrl">
    <button type="button" ng-click="btn_click()">設定</button>
    <wj-calendar control="calendar" value="date"  display-month="month" style="width: 300px"></wj-calendar>
</div>