var date = new Date();
var firstDay = new Date(date.getFullYear(), date.getMonth(), 1);
var lastDay = new Date(date.getFullYear(), date.getMonth() + 1, 0);
var monthLastDay = Math.floor(lastDay.getTime()/1000);
console.log(monthLastDay);

1527705000
Is equivalent to:
05/30/2018 @ 6:30pm (UTC)