Find the Nth Day in Month of Selected Weekday

Month: January February March April May June July August September October November December Year: 1st 2nd 3rd 4th 5th Sun Mon Tue Wed Thu Fri Sat

Javascript function

	
function nth_zday(nth, zday, fdate) {
	// nth: 1=1st, 2=2nd, 3=3rd, 4=4th, 5=5th
	// zday: 0=Sun, 1=Tue, ..., 6=Sat
	// fdate: null (curr date) or date of interest
	fdate = fdate || new Date();

	// get day of week for first day of given month
	// calculate first zday
	// add in the number of weeks forward
	fdate.setDate(1);
	var fom = fdate.getDay();
	var monthDay = ((7 + zday - fom)%7) + ((nth-1)*7) + 1;

	// check for day > end of month
	// this utilizes Date()'s processing of days past the end of the month
	// Date(2018,08,36) => 2018/09/05
	var valiDate = new Date(fdate.getFullYear(), fdate.getMonth(), monthDay);
	if (fdate.getMonth() == valiDate.getMonth()) {
		return monthDay
	} else {
		return false;
	}
}