Helper Date Functions, including determining leap year, and how many days in a month

dateHelpers.js

var isLeapYear = function(year) {
    return year % 4 === 0 && year % 100 !== 0 || year % 400 === 0;
  }
  , getDaysInMonth = function(month, year) {
    return [31, isLeapYear(year) ? 29 : 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month];
  }
;