JavaScript date : How to get month and year difference between two dates?

function yearsDiff(d1, d2) {
    var date1 = new Date(d1);
    var date2 = new Date(d2);
    var yearsDiff =  date2.getFullYear() - date1.getFullYear();
    return yearsDiff;
}

function monthsDiff(d1, d2) {
  var date1 = new Date(d1);
  var date2 = new Date(d2);
  var years = yearsDiff(d1, d2);
  var monthsDiff =(years * 12) + (date2.getMonth() - date1.getMonth()) ;
  return monthsDiff;
}

JavaScript date : How to get date for first, mid and last day of a month?

var firstDayStartDate = new Date(anyDateOfTheMonth.getFullYear(), anyDateOfTheMonth.getMonth(), 1);

var midDayStartDate = new Date(anyDateOfTheMonth.getFullYear(), anyDateOfTheMonth.getMonth(), 15);

var midDayPlusOneStartDate = new Date(anyDateOfTheMonth.getFullYear(), anyDateOfTheMonth.getMonth(), 16);

var lastDayStartDate = new Date(anyDateOfTheMonth.getFullYear(), anyDateOfTheMonth.getMonth() + 1, 0);