Calculating working days between two dates with Javascript

Here’s a script that will calculate the number of working days between two Date objects in Javascript.  I had a hard time finding an accurate one online so I wrote one myself – please use with caution as it has not been rigorously tested.
Note that the calculation considers the difference between today and today to equal 1.  I’m using this to present the number of days left before an event, and in my case on the day of the event it makes sense to see “1 day” instead of “0 days.”  

function workingDaysBetweenDates(startDate, endDate) {
  
    // Validate input
    if (endDate < startDate)
        return 0;
    
    // Calculate days between dates
    var millisecondsPerDay = 86400 * 1000; // Day in milliseconds
    startDate.setHours(0,0,0,1);  // Start just after midnight
    endDate.setHours(23,59,59,999);  // End just before midnight
    var diff = endDate - startDate;  // Milliseconds between datetime objects    
    var days = Math.ceil(diff / millisecondsPerDay);
    
    // Subtract two weekend days for every week in between
    var weeks = Math.floor(days / 7);
    days = days - (weeks * 2);

    // Handle special cases
    var startDay = startDate.getDay();
    var endDay = endDate.getDay();
    
    // Remove weekend not previously removed.   
    if (startDay - endDay > 1)         
        days = days - 2;      
    
    // Remove start day if span starts on Sunday but ends before Saturday
    if (startDay == 0 && endDay != 6)
        days = days - 1  
            
    // Remove end day if span ends on Saturday but starts after Sunday
    if (endDay == 6 && startDay != 0)
        days = days - 1  
    
    return days;
}

JSFiddle