fix(datepicker): check if initDate is valid

Closes #5190
Closes #5266

Author: AndriIushchuk

Diff for: src/datepicker/datepicker.js

@@ -228,10 +228,12 @@ angular.module('ui.bootstrap.datepicker', ['ui.bootstrap.dateparser', 'ui.bootst
         $log.warn('uib-datepicker initDate attribute usage is deprecated, use datepicker-options attribute instead');
       }
 
-      this.activeDate = dateParser.fromTimezone($scope.$parent.$eval($attrs.initDate), ngModelOptions.timezone) || new Date();
+      var initDate = dateParser.fromTimezone($scope.$parent.$eval($attrs.initDate), ngModelOptions.timezone);
+      this.activeDate = !isNaN(initDate) ? initDate : new Date();
       watchListeners.push($scope.$parent.$watch($attrs.initDate, function(initDate) {
         if (initDate && (ngModelCtrl.$isEmpty(ngModelCtrl.$modelValue) || ngModelCtrl.$invalid)) {
-          self.activeDate = dateParser.fromTimezone(initDate, ngModelOptions.timezone);
+          initDate = dateParser.fromTimezone(initDate, ngModelOptions.timezone);
+          self.activeDate = !isNaN(initDate) ? initDate : new Date();
           self.refreshView();
         }
       }));