JSFiddle - React, Tailwind, and code Playground

HTML

<label for="rabotaZapochva" style="display:inline;">РАБОТА ЗАПОЧВА:</label>
<select name="rabotaZapochva" id="rabotaZapochva" style="display: inline;width: 90px;font-size: 19px;font-weight: bold;">
                           ...

JavaScript

$(document).ready(function() {
  $("#rabotaZapochva").change(workingAtNight);
  $("#rabotaShvyrshva").change(workingAtNight);
});

function workingAtNight () {
        var start_time = $('#rabotaZapochva').val();
        var end_time = $('#rabotaShvyrshva').val();
        if(end_time > "06:00" && end_time < "22:00"){
            end_time = "06:00";
        }


        if((end_time > "22:00") || (end_time < "06:00")){
            if(start_time > "22:00" || (start_time > "00:00" && start_time < "06:00")){
                start_time = $('#rabotaZapochva').val();
            }
            else{
                start_time = "22:00";
            }
        }
        else{
            start_time = $('#rabotaZapochva').val();
        }


        var diff = ( new Date("1970-1-1 " + end_time) - new Date("1970-1-1 " + start_time) ) / 60000 / 60;
        
        if(diff > 8){
            diff = diff - 16;
        }

        var sumaNoshtenTrud = 0;

        var decimalTimeString = diff;
        var n = new Date(0,0);
        n.setSeconds(+decimalTimeString * 60 * 60);
        var test = n.toTimeString().slice(0, 5);
        if(test > "08:00"){
            test = "08:00";
        }


        if(start_time >= "06:00" && start_time < "22:00"){
            test = "00:00";
        }



        $('#noshtenTrud').val(test);
        
        

    };