Calculate how many more builds you can create today on a free Codeship account in order to not be overstepping your average daily allotment

index.js

/**
 * This function gratefully taken from
 * http://stackoverflow.com/a/1185804/1591507
 */
Date.prototype.monthDays = () => {
  const date = new Date();
  const d = new Date(date.getFullYear(), date.getMonth() + 1, 0);
  return d.getDate();
}

/**
 * @param {string} numberofBuildsDone - the number of builds 
 * that you’ve already used for the current month
 */
const calc = (numberofBuildsDone) => {
  const numberFreeBuildsPerMonth = 100;
  const daysInMonth = new Date().monthDays();
  const currentDayInMonth = new Date().getDate();
  const availableBuildsPerDay = numberFreeBuildsPerMonth / daysInMonth;
  const numBuildsLeftForToday = Math.round((availableBuildsPerDay * currentDayInMonth) - numberofBuildsDone);
  return `You can make ${numBuildsLeftForToday} more builds today.`;
}