javascript date calculations ignore weekends
function getBusinessDateCount (startDate, endDate) {
var elapsed, daysBeforeFirstSaturday, daysAfterLastSunday;
var ifThen = function (a, b, c) {
return a == b ? c : a;
};
elapsed = endDate - startDate;
elapsed /= 86400000;
daysBeforeFirstSunday = (7 - startDate.getDay()) % 7;
daysAfterLastSunday = endDate.getDay();
elapsed -= (daysBeforeFirstSunday + daysAfterLastSunday);
elapsed = (elapsed / 7) * 5;
elapsed += ifThen(daysBeforeFirstSunday - 1, -1, 0) + ifThen(daysAfterLastSunday, 6, 5);
return Math.ceil(elapsed);
}
function calc() {
let start = document.querySelector('#startDate').value,
end = document.querySelector('#endDate').value,
result = getBusinessDateCount(new Date(start), new Date(end));
document.querySelector('#result').value = result;
}