var
   MonthDays=new Array();

   MonthDays[false]=new Array( 31 , 28 , 31 , 30 , 31 , 30 , 31 , 31 , 30 , 31 , 30 , 31 );
   MonthDays[true]=new Array( 31 , 29 , 31 , 30 , 31 , 30 , 31 , 31 , 30 , 31 , 30 , 31 );

function IsLeapYear(aYear)
{
    return(((aYear% 4 == 0 ) && (aYear% 100 != 0 )) || (aYear% 400 == 0 ));
}

function IncDay(aDate,aIncValue)
{
    if(!(aDate instanceof Date))
      return(NaN);

    if(arguments.length== 1 )
      aIncValue= 1 ;

    return(new Date(aDate.getTime()+ 1000 * 60 * 60 * 24 *aIncValue));
}

function IncMonth(aDate,aIncValue)
{
    var
      sign,
      leapyear,
      day,
      month,
      year;

    if(!(aDate instanceof Date))
      return(NaN);

    if(arguments.length== 1 )
      aIncValue= 1 ;

    day=aDate.getDate();
    month=aDate.getMonth();
    year=aDate.getFullYear();

    sign = aIncValue>= 0  ?  1  : - 1 ;
    year+=(aIncValue-aIncValue% 12 )/ 12 ;
    aIncValue%= 12 ;
    month+=aIncValue;
    if(month >  11  || month< 0 )
    {
       year+=sign;
       month+=- 12 *sign;
    }
    leapyear=IsLeapYear(year);
    if(day>MonthDays[leapyear][month])
      day=MonthDays[leapyear][month];

    return(new Date(year,month,day));
}

function IncYear(aDate,aIncValue)
{
    if(!(aDate instanceof Date))
      return(NaN);

    if(arguments.length== 1 )
      aIncValue= 1 ;

    return(IncMonth(aDate, 12 *aIncValue));
}