SO | appointment

JavaScript

function appointment(srcDate) {
	console.log('');
	console.log(srcDate);
  
  var today = new Date();
  var todayNextMonth = today.getMonth() + 1;
  todayNextMonth = todayNextMonth > 11 ? 0 : todayNextMonth;

  if (srcDate < today) {
    console.log("Due");
  } else if (srcDate.getMonth() === today.getMonth()) {
    console.log("Due this month");
  } else if (srcDate.getMonth() === todayNextMonth) {
    console.log("Due next month");
  }
}

appointment(new Date("2016-08-31T00:00:00"));
appointment(new Date("2018-05-29T00:00:00"));
appointment(new Date("2018-06-15T00:00:00"));