Source: differenceInDays/index.js

import toDate from '../toDate/index'
import differenceInCalendarDays from '../differenceInCalendarDays/index'
import requiredArgs from '../_lib/requiredArgs/index'

// Like `compareAsc` but uses local time not UTC, which is needed
// for accurate equality comparisons of UTC timestamps that end up
// having the same representation in local time, e.g. one hour before
// DST ends vs. the instant that DST ends.
function compareLocalAsc(dateLeft, dateRight) {
  var diff =
    dateLeft.getFullYear() - dateRight.getFullYear() ||
    dateLeft.getMonth() - dateRight.getMonth() ||
    dateLeft.getDate() - dateRight.getDate() ||
    dateLeft.getHours() - dateRight.getHours() ||
    dateLeft.getMinutes() - dateRight.getMinutes() ||
    dateLeft.getSeconds() - dateRight.getSeconds() ||
    dateLeft.getMilliseconds() - dateRight.getMilliseconds()

  if (diff < 0) {
    return -1
  } else if (diff > 0) {
    return 1
    // Return 0 if diff is 0; return NaN if diff is NaN
  } else {
    return diff
  }
}

/**
 * @name differenceInDays
 * @category Day Helpers
 * @summary Get the number of full days between the given dates.
 *
 * @description
 * Get the number of full day periods between two dates. Fractional days are
 * truncated towards zero.
 *
 * One "full day" is the distance between a local time in one day to the same
 * local time on the next or previous day. A full day can sometimes be less than
 * or more than 24 hours if a daylight savings change happens between two dates.
 *
 * To ignore DST and only measure exact 24-hour periods, use this instead:
 * `Math.floor(differenceInHours(dateLeft, dateRight)/24)|0`.
 *
 *
 * ### v2.0.0 breaking changes:
 *
 * - [Changes that are common for the whole library](https://github.com/date-fns/date-fns/blob/master/docs/upgradeGuide.md#Common-Changes).
 *
 * @param {Date|Number} dateLeft - the later date
 * @param {Date|Number} dateRight - the earlier date
 * @returns {Number} the number of full days according to the local timezone
 * @throws {TypeError} 2 arguments required
 *
 * @example
 * // How many full days are between
 * // 2 July 2011 23:00:00 and 2 July 2012 00:00:00?
 * var result = differenceInDays(
 *   new Date(2012, 6, 2, 0, 0),
 *   new Date(2011, 6, 2, 23, 0)
 * )
 * //=> 365
 * // How many full days are between
 * // 2 July 2011 23:59:00 and 3 July 2011 00:01:00?
 * var result = differenceInDays(
 *   new Date(2011, 6, 3, 0, 1),
 *   new Date(2011, 6, 2, 23, 59)
 * )
 * //=> 0
 * // How many full days are between
 * // 1 March 2020 0:00 and 1 June 2020 0:00 ?
 * // Note: because local time is used, the
 * // result will always be 92 days, even in
 * // time zones where DST starts and the
 * // period has only 92*24-1 hours.
 * var result = differenceInDays(
 *   new Date(2020, 5, 1),
 *   new Date(2020, 2, 1)
 * )
//=> 92
 */
export default function differenceInDays(dirtyDateLeft, dirtyDateRight) {
  requiredArgs(2, arguments)

  var dateLeft = toDate(dirtyDateLeft)
  var dateRight = toDate(dirtyDateRight)

  var sign = compareLocalAsc(dateLeft, dateRight)
  var difference = Math.abs(differenceInCalendarDays(dateLeft, dateRight))

  dateLeft.setDate(dateLeft.getDate() - sign * difference)

  // Math.abs(diff in full days - diff in calendar days) === 1 if last calendar day is not full
  // If so, result must be decreased by 1 in absolute value
  var isLastDayNotFull = compareLocalAsc(dateLeft, dateRight) === -sign
  var result = sign * (difference - isLastDayNotFull)
  // Prevent negative zero
  return result === 0 ? 0 : result
}