[Javascript] VBScript DateAdd 함수 구현

/**
 * 
 * @param interval : yyyy(년), m(월), d(일)
 * @param number : 증가 년, 월, 일
 * @param dateformat : 날짜 문자열
 * @returns {String} yyyymmdd 8자리로 리턴
 */
function DateAdd(interval, number, dateformat) {
  dateformat = clear_DateSTRING(dateformat);
  var int_millisecond = 1;
  var int_second      = 1000 * int_millisecond;
  var int_minute      = 60 * int_second;
  var int_hour        = 60 * int_minute;
  var int_day         = 24 * int_hour;
  
  var YY_form = CInt(Left(dateformat, 4));
  var MM_form = CInt(Mid(dateformat, 5, 2))-1;
  var DD_form = CInt(Right(dateformat, 2));
  
  var date = new Date(YY_form, MM_form, DD_form);
  var date_milliseconds = date.valueOf();
  var add_milliseconds  = 0;
  
  var ret_date = null;
  switch (interval) {
  case "yyyy":
    date.setFullYear(date.getFullYear()+number, date.getMonth(), date.getDate());
    ret_date = date;
    break;
  case "m":
    date.setFullYear(date.getFullYear(), date.getMonth()+number, date.getDate());
    ret_date = date;
    break;
  case "d":
    add_milliseconds  = number * int_day;
    ret_date = new Date(date_milliseconds + add_milliseconds);
    break;
  }
  
  var year  = ret_date.getFullYear();
  var month = ret_date.getMonth() + 1;
  if ( month < 10 ) {
    month = "0" + month;
  }
  var day   = ret_date.getDate();
  if ( day < 10 ) {
    day = "0" + day;
  }
  
  return ( "" + year + month + day );	
}