DateDiff.js

let startDate = new Date("2010-05-28");
let endDate = new Date("2020-03-01");

function isDateValid(dateToCheck) {
  return isNaN(Date.parse(dateToCheck)) ? false : true;
}

function getMilliSecDiff(startDate, endDate) {
  let startMilliSec = startDate.getTime();
  let endMilliSec = endDate.getTime();
  return endMilliSec - startMilliSec;
}

function getSecondDiff(startDate, endDate) {
  let milisecsDiff = getMilliSecDiff(startDate, endDate);
  // 1 sec = 1000ms
  return milisecsDiff / 1000;
}

function getMinuteDiff(startDate, endDate) {
  let secondDiff = getSecondDiff(startDate, endDate);
  // 1 min = 60sec
  return ~~(secondDiff / 60);
}

function getHourDiff(startDate, endDate) {
  let hourDiff = getMinuteDiff(startDate, endDate);
  // 1 hour = 60mins
  return ~~(hourDiff / 60);
}

function getDayDiff(startDate, endDate) {
  let dayDiff = getHourDiff(startDate, endDate);
  // 1 Day = 24hour
  return ~~(dayDiff / 24);
}

function getWeekDiff(startDate, endDate) {
  let weekDiff = getDayDiff(startDate, endDate);
  // 1 Day = 24hour
  return ~~(weekDiff / 7);
}

console.log(getMilliSecDiff(startDate, endDate));
console.log(getSecondDiff(startDate, endDate));
console.log(getMinuteDiff(startDate, endDate));
console.log(getHourDiff(startDate, endDate));
console.log(getWeekDiff(startDate, endDate));
console.log(getDayDiff(startDate, endDate));

// function getYearDiff(fromDate, toDate) {}

function getMonthDiff_v1(startDate, endDate) {
  if (!isDateValid(startDate) && !isDateValid(endDate))
    return "Date(s) is/are not valid";
  if (endDate < startDate) {
    return "End Date Date is less than To Start Date";
  }
  let monthsFromYearDiff =
    (endDate.getFullYear() - startDate.getFullYear()) * 12;
  let monthsFromMonthDiff = endDate.getMonth() - startDate.getMonth();
  let dayDiff = endDate.getDate() - startDate.getDate();
  if (dayDiff < 0) {
    // means fromDate is not a complete month
    return monthsFromYearDiff + monthsFromMonthDiff - 1;
  }
  return monthsFromYearDiff + monthsFromMonthDiff;
}

console.log(getMonthDiff_v1(startDate, endDate));

function getMonthDiff_v2(startDate, endDate) {
  var diff = (endDate.getTime() - startDate.getTime()) / 1000;
  diff /= (60 * 60 * 24 * 365) / 12;
  return Math.abs(Math.round(diff));
}

console.log(getMonthDiff_v2(startDate, endDate));

function getMonthDiff_v3(startDate, endDate) {
  let diff = (endDate.getTime() - startDate.getTime()) / 1000;
  // 1 month = 4 week
  // 1 week = 7 days
  // 1 day= (60 * 60 * 24) hours
  diff /= 60 * 60 * 24 * 7 * 4;
  return Math.abs(Math.floor(diff));
}

console.log(getMonthDiff_v3(startDate, endDate));

function getMonthDayDiff(startDate, endDate) {
  if (!isDateValid(startDate) && !isDateValid(endDate))
    return "Date(s) is/are not valid";
  if (endDate < startDate) {
    return "End Date Date is less than To Start Date";
  }
  let monthsFromYearDiff =
    (endDate.getFullYear() - startDate.getFullYear()) * 12;
  let monthsFromMonthDiff = endDate.getMonth() - startDate.getMonth();
  let dayDiff = endDate.getDate() - startDate.getDate();
  if (dayDiff < 0) {
    // means fromDate is not a complete month
    var dtmp = new Date(startDate.getFullYear(), startDate.getMonth() + 1, 0);
    var numDays = dtmp.getDate();
    dayDiff += numDays;
    let month = monthsFromYearDiff + monthsFromMonthDiff - 1;
    return { month, dayDiff };
  }
  let month = monthsFromYearDiff + monthsFromMonthDiff;
  return { month, dayDiff };
}

console.log(getMonthDayDiff(startDate, endDate));