When i click custom button in fullCalendar v4 datesRender call does not work

Question

when i change month intervals function in datesRender work perfectly but when i click booknow button and then change month on my calender it does not call intervals. i have added code you can check if there is any solution let me know

calendar = new Calendar(calendarEl, {
                customButtons: {
                    booknow: {
                    text: 'Confirm Booking',
                    click: function() {
                        booking_payment();
                    }
                    }
                },
            height: 650,
            plugins: ['bootstrap', 'interaction', 'dayGrid', 'timeGrid'],
            // defaultView: 'timeGridWeek',
            selectable: true,
            // selectHelper: true,
            unselectAuto: false,
            header: {
                left: 'prev,next today booknow',
                center: 'title',
                right: 'dayGridMonth,timeGridWeek,timeGridDay'
            },
                eventRender: function(info) {
                var tooltip = new Tooltip(info.el, {
                    title: info.event.extendedProps.description,
                    placement: 'top',
                    trigger: 'hover',
                    container: 'body'
                });
                },  
                datesRender: function(info){     
                    if($process.length!=0)
                    {
                        intervals();
                    }
                },  
            events: "{{ url('/student/tutor/'.$tutor->id.'/booking') }}",
            showNonCurrentDates: false,

            // editable: true,
            droppable: true, // this allows things to be dropped onto the calendar !!!
            drop: function (info) {
                // is the "remove after drop" checkbox checked?
                if (checkbox.checked) {
                    // if so, remove the element from the "Draggable Events" list
                    info.draggedEl.parentNode.removeChild(info.draggedEl);
                }
            },

            dayClick: function(date, jsEvent, view) { 
                console.log('Clicked on: ' + date.getDate()+"/"+date.getMonth()+"/"+date.getFullYear());  
            },

        });

        calendar.render();

here is booking payment method it calculates all pending process and show their summary on a modal name exampleModal a simple modal with payment preview from the pending process

function booking_payment()
    {
        $('#exampleModal').modal("show");
            $values="<table style='border:4px'><tr><th>Subject</th><th>Time Slot </th><th>Amount</th>"
            $process.forEach(myFunction);
            $values+="<tr><td></td><td>Total Amount </td><td> " + $totalprice+"</td></tr></table>"
        $('#show_processing').html($values)
        }
        function myFunction(item, index) {
            $values=$values+"<tr><td>"+ item[0] +" </td><td> "+item[1]+" "+item[3]+" - "+item[2]+" "+item[4]+" </td><td> "
            item[4]=item[4].replace(':', '.')
            item[3]=item[3].replace(':', '.')
            $values=$values .concat(Math.floor(((parseFloat(item[4])-parseFloat(item[3]))*$price_per_hour * 100 / 100) * 100)/100)
            $values+="</td></tr>"
            $totalhours+=(parseFloat(item[4])-parseFloat(item[3])* 100) / 100
            $totalhours=Math.floor($totalhours * 100) / 100
            $totalprice+=((parseFloat(item[4])-parseFloat(item[3]))*$price_per_hour * 100) / 100
            $totalprice=Math.floor($totalprice * 100) / 100
                }

Here is code for intervals it takes all events from a given view and checks if we have any pending process in given time so it replaces the event.

function intervals()
    {
        var myVar = setInterval(function (){
               var events_this_date=calendar.getEvents()
        events_this_date.forEach(events_remove_fun);
            }, 50);

        setTimeout(function(){
            clearInterval(myVar);
                }, 10000);           
    }

       function events_remove_fun(item, index) {
        if (item.id.substring(0,6) == "Booked")
                {
                    console.log("Already Booked");
                }
            else if (item.id.substring(0,9) == "c_pending")
                {
                    console.log("pending");
                }
                else if(item.id.substring(0,7) == "c_avail")
                {
                    console.log("available added by user");
                }
            else    
            {   
               var event_start=new Date(item.start).valueOf();
               var event_end=new Date(item.end).valueOf();
               var item_id=item.id;
               $process.forEach(function(item, index){
                $osd=item[1]+" "+item[3];
                $oed=item[1]+" "+item[4];
                $osd=$osd.replace("-", "/");
                $osd=$osd.replace("-", "/");
                $oed=$oed.replace("-", "/");
                $oed=$oed.replace("-", "/");
            if(new Date($osd).valueOf()>= event_start && new Date($oed).valueOf()<= event_end)
                {
                    var event = calendar.getEventById(item_id);
                    event.remove();
                    console.log(item_id);
                }
            });
           }
        }