Count the difference between two dates

countDateDifference.js

/**
 * Returns true if year is a leap year
 * @param {Number} year
*/
function isLeapYear(year) {
  return (((year % 4 == 0) && (year % 100 !== 0)) || (year % 400 == 0))
}

/**
 * Returns number of days till current month from start of the year
 * @param {Number} month
 * @param {Number} year
*/
function countDaysTillCurrentMonth(month, year) {
  let days = 0
  let monthDayCount = [
    31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
  ]

  for (let i = 0; i < month - 1; i++) {
    days += monthDayCount[i]
  }
  return days + ((month > 2) && isLeapYear(year) ? 1 : 0)
}

/**
 * Returns number of leap years between two years
 * @param {Number} from
 * @param {Number} to 
*/
function numberOfLeapYears(from, to) {
  let i = from
  let leapCount = 0

  while (i < to) {
    if (isLeapYear(i)) {
      leapCount++
      i += 4
    } else {
      i++
    }
  }

  return leapCount
}

/**
 * Returns difference of days between two years
 * @param {Number} from
 * @param {Number} to
*/
function daysBetweenYears(from, to) {
  if (from === to)
    return 0

  return ((to - from) * 365) + numberOfLeapYears(from, to)
}

/**
 * Returns the difference between two dates
 * @param {String} from - date in following format 'YYYY-MM-DD'
 * @param {String} to - date in following format 'YYYY-MM-DD'
*/
function countDateDifference(from, to) {
  let startDate = {
    year: parseInt(from.split('-')[0]),
    month: parseInt(from.split('-')[1]),
    day: parseInt(from.split('-')[2])
  }

  let endDate = {
    year: parseInt(to.split('-')[0]),
    month: parseInt(to.split('-')[1]),
    day: parseInt(to.split('-')[2])
  }

  let numberOfDaysTillStartDate = countDaysTillCurrentMonth(startDate.month, startDate.year) + startDate.day
  let numberOfDaysTillEndDate = countDaysTillCurrentMonth(endDate.month, endDate.year) + endDate.day + daysBetweenYears(startDate.year, endDate.year)

  return Math.abs(numberOfDaysTillEndDate - numberOfDaysTillStartDate - 1)
}

// Test Cases
console.log('Test countDateDifference - start')
console.log(countDateDifference('2020-09-25', '2020-09-28') === 2) // true
console.log(countDateDifference('2020-09-25', '2020-09-29') === 3) // true 
console.log(countDateDifference('2000-01-01', '2000-12-31') === 364) //  true
console.log(countDateDifference('2000-12-30', '2001-01-02') === 2) // true 
console.log(countDateDifference('1999`-12-30', '2001-01-02') === 368) // true 
console.log('Test countDateDifference - End')