JSFiddle - React, Tailwind, and code Playground

HTML

<h1>Stunden</h1>

<br>
<form name="jump" method="post" action="">Tag1 ... Zeit von:
    <select id="von_01" onChange="change('von_01', 'bis_01', 'hrs_01');">
        <option value="00:00">00:00</option>
        <option value="00:30">00:30</option>
        <option value="01:00">01:00</option>
        <option value="01:30">01:30</option>
        <option value="02:00">02:00</option>
        <option value="02:30">02:30</option>
        <option value="03:00">03:00</option>
        <option value="03:30">03:30</option>
        <option value="04:00">04:00</option>
        <option value="04:30">04:30</option>
        <option value="05:00">05:00</option>
        <option value="05:30">05:30</option>
        <option value="06:00">06:00</option>
        <option value="06:30">06:30</option>
        <option value="07:00">07:00</option>
        <option value="07:30">07:30</option>
        <option value="08:00">08:00</option>
        <option value="08:30">08:30</option>
        <option value="09:00">09:00</option>
        <option value="09:30">09:30</option>
        <option value="10:00">10:00</option>
        <option value="10:30">10:30</option>
        <option value="11:00">11:00</option>
        <option value="11:30">11:30</option>
        <option value="12:00">12:00</option>
        <option value="12:30">12:30</option>
        <option value="13:00">13:00</option>
        <option value="13:30">13:30</option>
        <option value="14:00">14:00</option>
        <option value="14:30">14:30</option>
        <option value="15:00">15:00</option>
        <option value="15:30">15:30</option>
        <option value="16:00">16:00</option>
        <option value="16:30">16:30</option>
        <option value="17:00">17:00</option>
        <option value="17:30">17:30</option>
        <option value="18:00">18:00</option>
        <option value="18:30">18:30</option>
        <option value="19:00">19:00</option>
        <option value="19:30">19:30</option>
        <option...

CSS

body {
    text-align:left;
}

JavaScript

var zeit = 0;
var von, bis;

function change(von, bis, ergebnis) {
    var value1 = document.getElementById(von).options[document.getElementById(von).selectedIndex].value;
    var value2 = document.getElementById(bis).options[document.getElementById(bis).selectedIndex].value;
    value1 = value1.substr(0, 2) * 60 + value1.substr(3, 2) * 1;
    value2 = value2.substr(0, 2) * 60 + value2.substr(3, 2) * 1;
    
    if (value1 == value2) {
        zeit = 24 * 60;
    } else if (value1 > value2) {
        zeit = (((24 - (value1/60)) + (value2/60)) * 60);
    } else {
        zeit = value2 - value1;
    }
    
    var hrs = (zeit / 60); 
    
    document.getElementById(ergebnis).value = hrs
}