Excel Date value ( DATEVALUE() ) Calculation in javascript

getExcelDateValue.js

//Calculate Excel Date value 
// Date 1899/12/31 serial number is 1
console.log(getExcelDateValue(new Date(1400,5,8)));//1400/6/8
console.log(getExcelDateValue(new Date(1899,11,31)));//1899/12/31
console.log(getExcelDateValue(new Date()));//now

function getExcelDateValue(dateObj){

  var year = dateObj.getFullYear();
  var month = dateObj.getMonth();
  var date = dateObj.getDate();
  var days = 1;//1899/12/31
  
  //days in the the month
  days += date;
  //days of months in the year
  while(month > 0){
    let d31 = [1,3,5,7,8,10,12];
    let d30 = [4,6,9,11];
    if(d31.includes(month)){
      days += 31;
    }else if(d30.includes(month)){
      days += 30;
    }else{
      days += isLeapYear(year) ? 29:28;
    }
    month -= 1;
  }
	
  if(year > 1899){///days from 1900 to the year before last
    year -= 1
    while(year > 1899){
      days += isLeapYear(year) ? 366:365;
      year -= 1;
    }
  }else{///days before 1900
  	let y = 1900
    while(year < y){
      days -= isLeapYear(y) ? 366:365;
      y -= 1;
    }
  }

  return days;

  function isLeapYear(year){
    let m400= year%400;
    let m100= year%100;
    let m4= year%4;
    return (m400 === 0 || ( m4 === 0 && m100 !== 0));
  }

}

see in JSFiddle :
https://jsfiddle.net/ShenTengTu/1h66aoef/