To get numbers of days in a month, use the getDate() method and get the days.
Example
You can try to run the following code to get a number of days in a month −
Live Demo
<!DOCTYPE html>
<html>
<body>
<script>
var days = function(month,year) {
return new Date(year, month, 0).getDate();
};
document.write("Days in July: "+days(7, 2012)); // July month
document.write("<br>Days in September: "+days(9, 2012)); // September Month
</script>
</body>
</html>Output
Days in July: 31 Days in September: 30