JSFiddle - React, Tailwind, and code Playground

HTML

Result: <span id="result"></span>

JavaScript

function funLoad(str1, str3, str4) {

    var dym1 = str1.split("/");
    
    // get the given(start) epoch value (using midnight for time - will adjust for actual time further below)
    var start_date = new Date(dym1[2], (dym1[1]-1), dym1[0], 0, 0, 0, 0);    
    var start_epoch = start_date.getTime();
    
    // get todays(end) epoch value (using midnight for time - will adjust for actual time further below)    
    var end_d = new Date();
    var end_epoch = new Date(end_d.getFullYear(), end_d.getMonth(), end_d.getDate(), 0, 0, 0, 0).getTime();
    var end_date_is_weekend = (end_d.getDay()==0 || end_d.getDay()==6)?1:0;    
    
    // get value for 1 day in milliseconds
    var one_day = 1000*60*60*24; 

    // how many days between given date and todays date
    var total_days = Math.ceil( (end_epoch - start_epoch ) / one_day)-1;
    
    // figure out how many of those total days are weekend days
    var date_to_check = start_epoch + one_day; 
    var total_weekend_days = 0;
    var next_day_is_weekend = 0;
    for(var x=0; x<total_days; x++){
        d=new Date(date_to_check);
        if(d.getDay()==0 || d.getDay()==6){
            total_weekend_days++;
            if(x==0){next_day_is_weekend=1;}
        }
        date_to_check += one_day
    }
    
    // determine number of week days
    var total_week_days = total_days - total_weekend_days;
  
    // get the total hours from the total week days
    var total_hours = total_week_days*24; 
    
    // still need to add the remaining hours from the given(start) date and the number of hours that have already past in the end(todays) date
    
    // add in the remaining hours left in the start date
    var hours_remaining_in_start_date = 0;
    
    // rounding up to next nearest start date hour
    if(str4 > 30){
        if(str3 == 23 && next_day_is_weekend==0){
            // rounding up puts the start date into the start of the next day so subtract
            // one day from the total, UNLESS the next...