label format in tooltip and datagrouping

Datagrouping let you format dates, but when zoomed in enough, the raw date has no format (see tooltip)

HTML

<script src="https://code.jquery.com/jquery-3.1.1.min.js"></script>
<script src="https://code.highcharts.com/stock/highstock.js"></script>

<div id="container" style="height: 400px; min-width: 310px"></div>

JavaScript

//create timedata
 var data = [];
 var m = Date.UTC(2016);
 var until = Date.now();

 while (m < until) {
 
   data.push({
     x: m,
     y: Math.floor(Math.random() * 1000)
   });
   m += 15 * 60 * 1000;
 }

 Highcharts.chart('container', {
   chart: {
     type: 'column',
     zoomType: 'x'
   },
   tooltip: {
   	 //headerFormat: "<span style='font-size: 10px'>{point.key}</span><br/>"
     //formatter: function() {
     //	console.log(this);
     //}
   },
   navigator: {
     enabled: true,
     height: 80,
     xAxis: {
       id: 'navAs',
       labels: {
         y: 10
       }
     }
   },
   plotOptions: {
     series: {
       type: 'column',
       turboThreshold: 0,
       dataGrouping: {
         enabled: true,

       }
     },
     column: {
       groupPadding: 0,
       pointPadding: 0,
       borderWidth: 1,
       crisp: true
     }
   },
   xAxis: {
     type: 'datetime',
     minRange: 6 * 3600 * 1000 // 6 hours
   },
   yAxis: [{
     title: {
       text: 'kWh'
     }
   }, {
     title: {
       text: 'm3'
     },
     opposite: true
   }],
   series: [{
     name: "Random",
     type: 'column',
     showInNavigator: true,
     yAxis: 0,
     data: data
   }]
 });