@kodian,

You can use formatDate() inside contentFormatter to format the date displayed in toolTip as shown in code snippet below –

toolTip: {
  contentFormatter: function (e) {
    var content = " ";
    for (var i = 0; i < e.entries.length; i++) {
      content += "<span style='color:" + e.chart.selectedColorSet[i] + ";'>" + CanvasJS.formatDate(e.entries[i].dataPoint.label, "DD/MM/YYYY HH:mm:ss") + "</span>" + ": " + e.entries[i].dataPoint.y;
      content += "<br/>";
    }
    return content;
  }
},

Also, please take a look at this updated JSFiddle for an example on formatting date displayed inside the toolTip.

___________
Indranil Deo
Team CanvasJS