highcharts在php中添加日期pointStart从日期开始？

I have managed to make a column chart appear with values from a php array, the values are total expenditure on a particular day on holiday (a travel budget app I am making). How can I make the series pointStart from a date (start of the holiday) which I have retreived from the database and stored in a php variable? and also make each interval a new day? I have made an attmept where the pointStart does not work.

var chart2 = new Highcharts.Chart({
    chart: {
        renderTo: 'chart2',
        type: 'column'
    },

    xAxis: {
        type: 'datetime'
    },

    series: [{
        name: 'Days of trip',
        data:  [<?php echo join($trip_total, ',') ?>],
        pointStart: Date.UTC(<?php echo $date1 ?>),
        pointInterval: 3600 * 1000 // one hour
    }]
});