JSFiddle - React, Tailwind, and code Playground

JavaScript

var totalDays = 0;
var fromDateText = '0001-07-01';
var toDateText = '2019-07-05';

// Parse date text
// Of course I considered using Date class, however it seems to be not able to set 0001 year... for example I write new Date(1, 1, 1), Date class returns 1901 year.
var splittedFromDate = fromDateText.split('-');
var fromDate = {
	year: parseInt(splittedFromDate[0]),
	month: parseInt(splittedFromDate[1]),
	day: parseInt(splittedFromDate[2])
};
var splittedToDate = toDateText.split('-');
var toDate = {
	year: parseInt(splittedToDate[0]),
	month: parseInt(splittedToDate[1]),
	day: parseInt(splittedToDate[2])
};

// Calculate totally days
for (var i = fromDate.year; i <= toDate.year; i++) {
	var year = i;
  
  for (var j = 1; j <= 12; j++) {
  	var month = j;
    
    if (year == fromDate.year && month < fromDate.month) {
			continue;
      
    } else if (year == toDate.year && month > toDate.month) {
      continue;
      
    } else {
    	var daysPerMonth = getDaysPerMonth(year, month);
      
      if (year == fromDate.year && month == fromDate.month) {
      	totalDays += daysPerMonth - fromDate.day;
        
      } else if (year == toDate.year && month == toDate.month) {
        totalDays += toDate.day;
        
      } else {
      	totalDays += daysPerMonth;
      }
    }
  }
}

console.log(totalDays); // 739081

function getDaysPerMonth(year, month) {
  if (month in [2, 4, 6, 9, 11]) {
  	if (month == 2) {
      if (isLeapYear(year)) {
        return 29;
        
      } else {
        return 28;
      }
      
    } else {
    	return 30;
    }
    
  } else {
  	return 31;
  }
}

function isLeapYear(year) {
  if (year % 400 == 0) {
    return true;
    
  } else if (year % 100 == 0) {
    return false;
  
  } else if (year % 4 == 0) {
    return true;
  
  } else { 
	  return false;
  }
}