customize highcharts tooltip to show datetime

You should use xDateFormat in tooltip for customizing your format date
http://api.highcharts.com/highcharts#tooltip.xDateFormat

sample:
tooltip: {
           xDateFormat: '%Y-%m-%d'
         },

You can use moment.js to get the values formatted, but Highcharts has it's own date formatting functionality which would be more idiomatic with Highcharts. It can be attached onto the tooltip option in the highcharts constructor like so:

        tooltip: {
            formatter: function() {
                return  '<b>' + this.series.name +'</b><br/>' +
                    Highcharts.dateFormat('%e - %b - %Y',
                                          new Date(this.x))
                + ' date, ' + this.y + ' Kg.';
            }
        }

You may want to also add the dateTimeLabelFormats object with the options you need for your dates, under the xAxis.

I did this example with your code

You can use {value:%Y-%m-%d} template filter.

For an example:

headerFormat: '<span style="font-size: 10px">{point.key:%Y-%m-%d}</span><br/>'