[ActionScript 3.0] AS3 获取某年某月的天数(Get number of days in a month)

function getNumberOfDays($year:int, $month:int):int
{
    var month:Date = new Date($year, $month + 1, 0);
    return month.date;
} 

//jan
trace(getNumberOfDays(2010, 0));  // traces 31

//feb
trace(getNumberOfDays(2010, 1)); // traces 28

//dec
trace(getNumberOfDays(2010, 11)); // traces 31