Calculates business days between two dates using moment.js

calculateBusinessDays.js

var moment = require('moment');

function calculateBusinessDays(firstDate, secondDate){
  //Initiallize variables
  var day1 = moment(firstDate);
  var day2 = moment(secondDate);
  var adjust = 0;
  
  if((day1.dayOfYear() === day2.dayOfYear()) && (day1.year() === day2.year())){
    return 0;
  }
  
  //Check if second date is before first date to switch
  if(day2.isBefore(day1)){
    day2 = moment(firstDate);
    day1 = moment(secondDate);
  }

  //Check if first date starts on weekends
  if(day1.day() === 6) { //Saturday
    //Move date to next week monday
    day1.day(8);
  } else if(day1.day() === 0) { //Sunday
    //Move date to current week monday
    day1.day(1);
  }

  //Check if second date starts on weekends
  if(day2.day() === 6) { //Saturday
    //Move date to current week friday
    day2.day(5);
  } else if(day2.day() === 0) { //Sunday
    //Move date to previous week friday
    day2.day(-2);
  }

  var day1Week = day1.week();
  var day2Week = day2.week();

  //Check if two dates are in different week of the year
  if(day1Week !== day2Week){
    //Check if second date's year is different from first date's year
    if (day2Week < day1Week){
      day2Week += day1Week;
    }
    //Calculate adjust value to be substracted from difference between two dates
    adjust = -2 * (day2Week - day1Week);
  }

  return day2.diff(day1, 'days') + adjust;
}

I believe the function should:

• include start and end date (so today "minus" yesterday equals two, given both are business days)
• to avoid side affects of user input, we should use start of the day (I just have the cast that I was testing this function at 23:50 and as New Date() creates Date at 01:00 AM, the difference was rounded to zero.
  Here is my solution in typescript:

    calculateBusinessDays(firstDate, secondDate){
        // EDIT : use of startOf
        let day1 = moment(firstDate).startOf('day');
        let day2 = moment(secondDate).startOf('day');
        // EDIT : start at 1
        let adjust = 1;

        if((day1.dayOfYear() === day2.dayOfYear()) && (day1.year() === day2.year())){
            return 0;
        }

        if(day2.isBefore(day1)){
            const temp = day1;
            day1 = day2;
            day2 = temp;
        }

        //Check if first date starts on weekends
        if(day1.day() === 6) { //Saturday
            //Move date to next week monday
            day1.day(8);
        } else if(day1.day() === 0) { //Sunday
            //Move date to current week monday
            day1.day(1);
        }

        //Check if second date starts on weekends
        if(day2.day() === 6) { //Saturday
            //Move date to current week friday
            day2.day(5);
        } else if(day2.day() === 0) { //Sunday
            //Move date to previous week friday
            day2.day(-2);
        }

        const day1Week = day1.week();
        let day2Week = day2.week();

        //Check if two dates are in different week of the year
        if(day1Week !== day2Week){
            //Check if second date's year is different from first date's year
            if (day2Week < day1Week){
                day2Week += day1Week;
            }
            //Calculate adjust value to be substracted from difference between two dates
            // EDIT: add rather than assign (+= rather than =)
            adjust += -2 * (day2Week - day1Week);
        }
        
        return day2.diff(day1, 'days') + adjust;
    }

This is super handy, thanks!!

Error when year is changed.
Eg: day1 = 1575381077000; Tuesday, December 3, 2019
day2= 1578059477000; Friday, January 3, 2020

response is 30; which is wrong

Thanks bro