为什么我不能得到dateobject.getMonth()的长度？

function getMonthLength(date) {
  var nextMonth = new Date(date.getFullYear(), date.getMonth() + 1, 1);
  var currentMonth = new Date(date.getFullYear(), date.getMonth(), 1);
  var monthLength = (nextMonth - currentMonth) / (1000 * 60 * 60 * 24);
  return monthLength;
}

var currentDate = new Date();
var monthLength = getMonthLength(currentDate);
console.log(monthLength);