index.js
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var startOfISOWeek = require('../start_of_iso_week/index.js')
var MILLISECONDS_IN_MINUTE = 60000
var MILLISECONDS_IN_WEEK = 604800000
/**
* @category ISO Week Helpers
* @summary Get the number of calendar ISO weeks between the given dates.
*
* @description
* Get the number of calendar ISO weeks between the given dates.
*
* ISO week-numbering year: http://en.wikipedia.org/wiki/ISO_week_date
*
* @param {Date|String|Number} dateLeft - the later date
* @param {Date|String|Number} dateRight - the earlier date
* @returns {Number} the number of calendar ISO weeks
*
* @example
* // How many calendar ISO weeks are between 6 July 2014 and 21 July 2014?
* var result = differenceInCalendarISOWeeks(
* new Date(2014, 6, 21),
* new Date(2014, 6, 6)
* )
* //=> 3
*/
function differenceInCalendarISOWeeks (dirtyDateLeft, dirtyDateRight) {
var startOfISOWeekLeft = startOfISOWeek(dirtyDateLeft)
var startOfISOWeekRight = startOfISOWeek(dirtyDateRight)
var timestampLeft = startOfISOWeekLeft.getTime() -
startOfISOWeekLeft.getTimezoneOffset() * MILLISECONDS_IN_MINUTE
var timestampRight = startOfISOWeekRight.getTime() -
startOfISOWeekRight.getTimezoneOffset() * MILLISECONDS_IN_MINUTE
// Round the number of days to the nearest integer
// because the number of milliseconds in a week is not constant
// (e.g. it's different in the week of the daylight saving time clock shift)
return Math.round((timestampLeft - timestampRight) / MILLISECONDS_IN_WEEK)
}
module.exports = differenceInCalendarISOWeeks