A snippet to know when is the next pay day

next_quincena.js

// Snippet to know when is the next pay day (quincena)
// The rules are:
// -Pay day is on the 15th and the last day of the month
// -- If that day is not a weekday (Mon-Fri) pay day falls on the previous Friday
// Of course, without additional effort this won't take into consideration holidays

// January is 0 and so on
var daysInMonth = [31,28,31,30,31,30,31,31,30,31,30,31];

function isLeapYear(year) {
  return ((year % 4 === 0) && (year % 100 !== 0)) || (year % 400 === 0);
}

function getDaysInMonth(month, year) {
  // Special case for February
  if (month == 1 && isLeapYear(year)) {
    return 29;
  }
  return daysInMonth[month];
}

// Gets the next 15th or last day of the month, whichever is closer
function getNextPayDate (d) {
  var dayOfTheMonth = d.getDate();
  if (dayOfTheMonth == 15) {
    return d;
  }
  if (dayOfTheMonth < 15) {
    d.setDate(15);
    return d;
  }
  var lastDayOfTheMonth = getDaysInMonth(d.getMonth(), d.getFullYear());
  d.setDate(lastDayOfTheMonth);
  return d;
}


function isWeekDay(day) {
  // Sunday is 0, Saturday is 6
  return day !== 0 && day !== 6;
}

function getEarliestFriday(d) {
  var dayOfTheWeek = d.getDay();
  if (dayOfTheWeek == 5) {
    return d;
  }
  // How many days do we have to go back?
  // alternative is (d - 5 + 7) % 7
  var daysToGoBack;
  if (dayOfTheWeek >= 5) {
    daysToGoBack = dayOfTheWeek - 5;
  } else {
    daysToGoBack = dayOfTheWeek + 2;
  }

  d.setDate(d.getDate() - daysToGoBack);
  return d;
}

function getNextQuincena(d) {
  var nextPayDate = getNextPayDate(d);
  if (!isWeekDay(nextPayDate.getDay())) {
    return getEarliestFriday(nextPayDate);
  }
  return nextPayDate;
}