javascript – Fill select option input depending on other form field-ThrowExceptions

I’m new here and new to php.
I searched for this exact problem, but couldn’t find anything proper for my problem.

I am writing a wordpress plugin where I am creating the working schedule for employees. I just finished my vacation tool where the employees can take their vacations. These gets written into a database table.

In the second tool where I am creating the schedule, I have a simple form with html, js and php where I am first selecting then date and time (as shown in the code below) and after that picking the employee from a dropdown list (also the code below). Now I want the employee only show up if he isn’t on vacation at this date from the first input field.

I know that php is server sided and can not solve this problem properly, so how could I manage this with js or even jquery?

Code for datetime selection: (just simple input)

<input type="datetime-local" name="date" required/>

Code for select option field:

<?php 

global $wpdb;
$result = $wpdb->get_results("SELECT * FROM wp_einteilung_referees WHERE license_type ='Hauptschiedsrichter'");
?>
        <select name="hsr1" class="require_3">
            <option value="" disabled selected>Select HSR1</option>
                <?php foreach($result as $value){ ?> 
                    <option value="<?php echo $value->lastname; ?>"><?php echo $value->lastname; ?></option>        
                <?php } ?>
        </select>

<script type="text/javascript">

$(document).ready(function(){

$('#dateid').on('change',function(){

    var seldate= $(this).val();


    if(seldate!=''){

        $.ajax({

            type:'POST',

            url:'pathtophppage.php',

            data:'var='+valuetoserver,

            success:function(html){

                        $('#id').html(html);


                 }

        }); 

    }

});