What is the best way to determine the number of days in a month with javascript?

function daysInMonth(iMonth, iYear) {
   return 32 - new Date(iYear, iMonth, 32).getDate();
}

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

var daysInMonth = (function() {
    var cache = {};
    return function(month,year) {
        var entry = year+"-"+month;
        if (entry in cache)
            return cache[entry];
        return cache[entry] = new Date(year, month, 0).getDate();
    }
})();