Source: differenceInCalendarDays/index.js

import getTimezoneOffsetInMilliseconds from '../_lib/getTimezoneOffsetInMilliseconds/index'
import startOfDay from '../startOfDay/index'
import requiredArgs from '../_lib/requiredArgs/index'

var MILLISECONDS_IN_DAY = 86400000

/**
 * @name differenceInCalendarDays
 * @category Day Helpers
 * @summary Get the number of calendar days between the given dates.
 *
 * @description
 * Get the number of calendar days between the given dates. This means that the times are removed
 * from the dates and then the difference in days is calculated.
 *
 * ### 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 calendar days
 * @throws {TypeError} 2 arguments required
 *
 * @example
 * // How many calendar days are between
 * // 2 July 2011 23:00:00 and 2 July 2012 00:00:00?
 * var result = differenceInCalendarDays(
 *   new Date(2012, 6, 2, 0, 0),
 *   new Date(2011, 6, 2, 23, 0)
 * )
 * //=> 366
 * // How many calendar days are between
 * // 2 July 2011 23:59:00 and 3 July 2011 00:01:00?
 * var result = differenceInCalendarDays(
 *   new Date(2011, 6, 3, 0, 1),
 *   new Date(2011, 6, 2, 23, 59)
 * )
 * //=> 1
 */
export default function differenceInCalendarDays(
  dirtyDateLeft,
  dirtyDateRight
) {
  requiredArgs(2, arguments)

  var startOfDayLeft = startOfDay(dirtyDateLeft)
  var startOfDayRight = startOfDay(dirtyDateRight)

  var timestampLeft =
    startOfDayLeft.getTime() - getTimezoneOffsetInMilliseconds(startOfDayLeft)
  var timestampRight =
    startOfDayRight.getTime() - getTimezoneOffsetInMilliseconds(startOfDayRight)

  // Round the number of days to the nearest integer
  // because the number of milliseconds in a day is not constant
  // (e.g. it's different in the day of the daylight saving time clock shift)
  return Math.round((timestampLeft - timestampRight) / MILLISECONDS_IN_DAY)
}