JSFiddle - React, Tailwind, and code Playground

JavaScript

function timeDiff(first, second) {
    if (first === '' || second === '') {
        return false;
    }
    var f = first.split(' '),
        s = second.split(' ');

    if (first == '12:00 AM') {
        f[0] = '0';
    }
    if (first == '12:00 PM') {
        f[1] = 'AM';
    }
    if (second == '12:00 AM') {
        s[0] = '24';
    }
    if (second == '12:00 PM') {
        s[1] = 'AM';
    }

    
    var time1 = f[0].split(':'),
        time2 = s[0].split(':');

    time1 = time1[0] * 3600 + time1[1] * 60;
    time2 = time2[0] * 3600 + time2[1] * 60;
    var td = time2 - time1,
        hours = parseInt((td / 3600), 10),
        minutes = parseInt(((td - hours * 3600) / 60), 10),
        diff = ((hours < 10 && hours >= 0) ? ('0' + hours) : hours) + ':' + ((minutes < 10 && minutes >= 0) ? ('0' + minutes) : minutes);
    return (diff);
}

var FirstTime = '1:00 AM';
var SecondTime = '12:09 pM';

alert(timeDiff(FirstTime, SecondTime));