Method to add some days to a date. Takes into account any timezone offset.

add-days

/**
 * Adds nb days to the specified date.
 * @param {Date} the date.
 * @param {Int} the number of days you want to add (or remove if negative).
 * @return {Date} the shifted date.
 */
var addDays = function(date, nb) {
  // day duration in ms.
  var DAY = 1000*60*60*24;
  
  // shifted date.
  var result = new Date(date.getTime() + nb * DAY);
  
  // timezone offset of date.
  var dateOffset = date.getTimezoneOffset();
  
  // timezone offset of the result.
  var resultOffset = result.getTimezoneOffset();
  
  //fixes the hour of the result due by a timezone difference.
  result = new Date(result.getTime() - (dateOffset- resultOffset)*60*1000);
  
  return result;
};