Datageek’s solution helped me but I needed to augment it. This still doesn’t do holidays but does do working days with the option of including Sat and/or Sun, and does support adding negative days:-
function AddWorkingDays(datStartDate, lngNumberOfWorkingDays, blnIncSat, blnIncSun) {
var intWorkingDays = 5;
var intNonWorkingDays = 2;
var intStartDay = datStartDate.getDay(); // 0=Sunday ... 6=Saturday
var intOffset;
var intModifier = 0;
if (blnIncSat) { intWorkingDays++; intNonWorkingDays--; }
if (blnIncSun) { intWorkingDays++; intNonWorkingDays--; }
var newDate = new Date(datStartDate)
if (lngNumberOfWorkingDays >= 0) {
// Moving Forward
if (!blnIncSat && blnIncSun) {
intOffset = intStartDay;
} else {
intOffset = intStartDay - 1;
}
// Special start Saturday rule for 5 day week
if (intStartDay == 6 && !blnIncSat && !blnIncSun) {
intOffset -= 6;
intModifier = 1;
}
} else {
// Moving Backward
if (blnIncSat && !blnIncSun) {
intOffset = intStartDay - 6;
} else {
intOffset = intStartDay - 5;
}
// Special start Sunday rule for 5 day week
if (intStartDay == 0 && !blnIncSat && !blnIncSun) {
intOffset++;
intModifier = 1;
}
}
// ~~ is used to achieve integer division for both positive and negative numbers
newDate.setTime(datStartDate.getTime() + (new Number((~~((lngNumberOfWorkingDays + intOffset) / intWorkingDays) * intNonWorkingDays) + lngNumberOfWorkingDays + intModifier)*86400000));
return newDate;
}