function daysInMonth(month,year) {
    return new Date(year, month, 0).getDate();
}
daysInMonth(7,2009); //31
daysInMonth(2,2009); //28
daysInMonth(2,2008); //29

function daysInMonth(Month, Year)
 {
     return 32 - new Date(Year, Month, 32).getDate();
 }

    daysInMonth(7,2009)
>>> 31

Date.prototype.monthDays= function(){
    var d= new Date(this.getFullYear(), this.getMonth()+1, 0);
    return d.getDate();
}

Date.getDaysInMonth(2009, 9)

// pass in any date as parameter anyDateInMonth
function daysInMonth(anyDateInMonth) {
    return new Date(anyDateInMonth.getYear(),
                    ++anyDateInMonth.getMonth(),
                    0).getDate();}

function daysInMonth(month,year) {
    return new Date(year, month, 0).getDate();
}