$("#datepicker").on("change paste keyup", function () {
    const form_date = $(this).val();
    const reformatted_date = form_date[3] + form_date[4] + "/" + form_date[0] + form_date[1] + form_date.slice(5,10);
    const current_datetime = new Date();
    const object_form = new Date(reformatted_date);
    if(current_datetime < object_form){
        alert("Niepoprawna data!")
        $(this).val('');
    }
});