ある年のある月の日数を取得します.

//      
function getMonthDays(myYear, myMonth){
	var year = parseInt(myYear);
	var month = parseInt(myMonth);
	var monthStartDate = dateToUnix(year + "/" + month + "/1");
	
	//12          1 1 
	if(month == 12){
		year++;
		month = 1;
	}
	
	var monthEndDate = dateToUnix(year + "/" + (month + 1) + "/1");
	var days = (monthEndDate - monthStartDate)/(1000 * 60 * 60 * 24);  
	return days;  
}