code: func (p Pages) GroupByExpiryDate(format string, order ...string) (PagesGroup, error) {
  	sorter := func(p Pages) Pages {
  		return p.ByExpiryDate()
  	}
  	formatter := func(p Page) string {
  		return p.ExpiryDate().Format(format)
  	}
  	return p.groupByDateField(sorter, formatter, order...)
  }
docs: // GroupByExpiryDate groups by the given page's ExpireDate value in
  // the given format and with the given order.
  // Valid values for order is asc, desc, rev and reverse.
  // For valid format strings, see https://golang.org/pkg/time/#Time.Format