function select_preview_month() {
  var now = new Date(2009,2,1);
  var first_day = new Date(now.getFullYear(), now.getMonth(), 1);
  var last_day  = new Date(now.getFullYear(), now.getMonth(), 0);
  alert(first_day);
  alert(last_day);
  alert(first_day.getDate() + '.' +  first_day.getMonth()  + '.' + first_day.getFullYear());
  alert(last_day.getDate()  + '.' +  first_day.getMonth()   + '.' + last_day.getFullYear());
// first_day.getMonth() is not an error !!!
}

function select_preview_month() {
            var now = new Date();
            first_day = new Date(now.getFullYear(), now.getMonth(), 1);
            last_day = new Date(now.getFullYear(), now.getMonth(), 0);
            alert(first_day);
            alert(last_day);
          }

Get first and last date of preview month

date

JavaScript