I am using Grantt chart of (highcharts.com) by default the start and end date key of the object accepting values using javascript date().gettime function. I want to pass date formats like '2021-09-20' so i can create json object from php backend i have created a ajax call so it fetch from a backend and appear the response data in series.

// THE CHART

<script type="text/javascript">
Highcharts.ganttChart('container', {
    chart: {
        styledMode: true
    },
    title: {
        text: 'Highcharts Gantt in Styled Mode'
    },
    subtitle: {
        text: 'Purely CSS-driven design'
    },
    xAxis: {
        min: today.getTime() - (2 * day),
        max: today.getTime() + (32 * day)
    },
    series: [{
        name: 'Project 1',
        data: [{
            name: 'Planning',
            id: 'planning',
            start: '2021-09-20',
            end: ''
        }, {
            name: 'Requirements',
            id: 'requirements',
            parent: 'planning',
            start: today.getTime(),
            end: today.getTime() + (5 * day)
        }, {
            name: 'Design',
            id: 'design',
            dependency: 'requirements',
            parent: 'planning',
            start: today.getTime() + (3 * day),
            end: today.getTime() + (20 * day)
        }, {
            name: 'Layout',
            id: 'layout',
            parent: 'design',
            start: today.getTime() + (3 * day),
            end: today.getTime() + (10 * day)
        }, {
            name: 'Graphics',
            parent: 'design',
            dependency: 'layout',
            start: today.getTime() + (10 * day),
            end: today.getTime() + (20 * day)
        }, {
            name: 'Develop',
            id: 'develop',
            start: today.getTime() + (5 * day),
            end: today.getTime() + (30 * day)
        }, {
            name: 'Create unit tests',
            id: 'unit_tests',
            dependency: 'requirements',
            parent: 'develop',
            start: today.getTime() + (5 * day),
            end: today.getTime() + (8 * day)
        }, {
            name: 'Implement',
            id: 'implement',
            dependency: 'unit_tests',
            parent: 'develop',
            start: today.getTime() + (8 * day),
            end: today.getTime() + (30 * day)
        }]
    }]
});
</script>