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  var date = new Date(2015, 0, 2);


  function getDateAgo(date, ago) {
    var newDate = date;
    return new Date(newDate - (ago * 86400000)).getDate()
  }

  function getLastDayOfMonth(year, month) {
    var date = new Date(year, month + 1, 0);
    return date.getDate();
  }

  console.log(getLastDayOfMonth(2012, 1)); // 2, (2 января 2014)