JSFiddle - React, Tailwind, and code Playground

HTML

<input id="start" value="09:42"> <!-- 08.00 PM -->
<input id="end" value="09:30"> <!-- 09.30 AM -->

<input id="diff">

JavaScript

var start = document.getElementById("start").value;
var end = document.getElementById("end").value;

function hourDiff(start, end) {
    start = start.split(":");
    end = end.split(":");
    
    var hours = end[0] - start[0];
    if(start[0] > end[0]) {
        hours = 24 + hours;
    }

    var minutes = end[1] - start[1];
    if(start[1] > end[1]) {
        minutes = 60 + minutes;
        if(hours == 0) {
            hours = 23;
        } else {
            hours--;
        }
        
    }
 
    return (hours < 9 ? "0" : "") + hours + ":" + (minutes < 9 ? "0" : "") + minutes;
}

document.getElementById("diff").value = hourDiff(start, end);