var addISOYears = require('../add_iso_years/index.js')

/**
 * @category ISO Week-Numbering Year Helpers
 * @summary Subtract the specified number of ISO week-numbering years from the given date.
 *
 * @description
 * Subtract the specified number of ISO week-numbering years from the given date.
 *
 * ISO week-numbering year: http://en.wikipedia.org/wiki/ISO_week_date
 *
 * @param {Date|String|Number} date - the date to be changed
 * @param {Number} amount - the amount of ISO week-numbering years to be subtracted
 * @returns {Date} the new date with the ISO week-numbering years subtracted
 *
 * @example
 * // Subtract 5 ISO week-numbering years from 1 September 2014:
 * var result = subISOYears(new Date(2014, 8, 1), 5)
 * //=> Mon Aug 31 2009 00:00:00
 */
function subISOYears (dirtyDate, dirtyAmount) {
  var amount = Number(dirtyAmount)
  return addISOYears(dirtyDate, -amount)
}

module.exports = subISOYears