Datepicker convert millisecond to date time

When the script runs, the document probably hasn't been drawn yet.

Try this:

var dateFormat = "M/D/YYYY, h:mm A";

$( document ).ready(()=>{ // or $( document ).ready(function(){
    $('.datepicker').each(function(i, e) {
        new Pikaday({
           field: e,
           format: dateFormat,
           disableDayFn: function (date) { 
               return moment(date).isAfter(new Date());
           },
           onOpen: function() {
               if (!this._o.field.value) {
                   this.setDate(new Date());
               }
           }
        });
        $(e).on('change', function(ev) {
            onDatePickerSelect(ev);
        });
    }); 
});

function setDateOnLoad() {
    $("#dateFromTimestamp").each(function(i, e) { 
        new Pikaday({
           field: e,
           format: dateFormat,
           value: new Date(1488484260000)                        
        });

    }); 
}

this will ensure that the code runs only after the doc is drawn