Calculate an expected delivery date (accounting for holidays) in business days using JavaScript?

I've adapted Mark Giblin's revised code to better deal with end of year dates and also U.S. federal holidays. See below...

function businessDaysFromDate(date,businessDays) {
  var counter = 0, tmp = new Date(date);
  while( businessDays>=0 ) {
    tmp.setTime( date.getTime() + counter * 86400000 );
    if(isBusinessDay (tmp)) {
      --businessDays;
    }
    ++counter;
  }
  return tmp;
}

function isBusinessDay (date) {
  var dayOfWeek = date.getDay();
  if(dayOfWeek === 0 || dayOfWeek === 6) {
    // Weekend
    return false;
  }

  holidays = [
    '12/31+5', // New Year's Day on a saturday celebrated on previous friday
    '1/1',     // New Year's Day
    '1/2+1',   // New Year's Day on a sunday celebrated on next monday
    '1-3/1',   // Birthday of Martin Luther King, third Monday in January
    '2-3/1',   // Washington's Birthday, third Monday in February
    '5~1/1',   // Memorial Day, last Monday in May
    '7/3+5',   // Independence Day
    '7/4',     // Independence Day
    '7/5+1',   // Independence Day
    '9-1/1',   // Labor Day, first Monday in September
    '10-2/1',  // Columbus Day, second Monday in October
    '11/10+5', // Veterans Day
    '11/11',   // Veterans Day
    '11/12+1', // Veterans Day
    '11-4/4',  // Thanksgiving Day, fourth Thursday in November
    '12/24+5', // Christmas Day
    '12/25',   // Christmas Day
    '12/26+1',  // Christmas Day
  ];

  var dayOfMonth = date.getDate(),
  month = date.getMonth() + 1,
  monthDay = month + '/' + dayOfMonth;

  if(holidays.indexOf(monthDay)>-1){
    return false;
  }

  var monthDayDay = monthDay + '+' + dayOfWeek;
  if(holidays.indexOf(monthDayDay)>-1){
    return false;
  }

  var weekOfMonth = Math.floor((dayOfMonth - 1) / 7) + 1,
      monthWeekDay = month + '-' + weekOfMonth + '/' + dayOfWeek;
  if(holidays.indexOf(monthWeekDay)>-1){
    return false;
  }

  var lastDayOfMonth = new Date(date);
  lastDayOfMonth.setMonth(lastDayOfMonth.getMonth() + 1);
  lastDayOfMonth.setDate(0);
  var negWeekOfMonth = Math.floor((lastDayOfMonth.getDate() - dayOfMonth - 1) / 7) + 1,
      monthNegWeekDay = month + '~' + negWeekOfMonth + '/' + dayOfWeek;
  if(holidays.indexOf(monthNegWeekDay)>-1){
    return false;
  }

  return true;
}

Thanks for your input guys, I had a long hard re-think over the approach I was making for this and came up with this little number...

var businessDays = 7, counter = 0; // set to 1 to count from next business day
while( businessDays>0 ){
    var tmp = new Date();
    var startDate = new Date();
    tmp.setDate( startDate .getDate() + counter++ );
    switch( tmp.getDay() ){
            case 0: case 6: break;// sunday & saturday
            default:
                businessDays--;
            }; 
}

The idea was to start with the business days and count backwards to zero for each day encountered that fell in to the range of a business day. This use of switch would enable a person to declare a day in the week as a non-business day, for example someone may not work on a monday, therefore the addition of case:1 would include a monday.

This is a simple script and does not take in to account public or bank holidays, that would be asking for a much more complex script to work with.

The result is a date that is set to the date of shipping, the user can then extract the date info in any format that they please, eg.

var shipDate = tmp.toUTCString().slice(1,15);

We have UI that defaults search inputs to last business day or a week-ago. Here's something that works both forward and backward.

// add (or subtract) business days to provided date
addBusinessDays = function (startingDate, daysToAdjust) {
    var newDate = new Date(startingDate.valueOf()),
        businessDaysLeft,
        isWeekend,
        direction;

    // Timezones are scary, let's work with whole-days only
    if (daysToAdjust !== parseInt(daysToAdjust, 10)) {
        throw new TypeError('addBusinessDays can only adjust by whole days');
    }

    // short-circuit no work; make direction assignment simpler
    if (daysToAdjust === 0) {
        return startingDate;
    }
    direction = daysToAdjust > 0 ? 1 : -1;

    // Move the date in the correct direction
    // but only count business days toward movement
    businessDaysLeft = Math.abs(daysToAdjust);
    while (businessDaysLeft) {
        newDate.setDate(newDate.getDate() + direction);
        isWeekend = newDate.getDay() in {0: 'Sunday', 6: 'Saturday'};
        if (!isWeekend) {
            businessDaysLeft--;
        }
    }
    return newDate;
};

It would be easy to pass in an optional holidays data structure and adjust for that as well.

However, generating a holidays data structure, well, that will take a little more effort and is specific not only to every country and region, but also to every organization.