"at the eleventh hour"到底是什麼時候？

"at the eleventh hour"到底是什麼時候？
提示：
The Internet had made it easier for airlines to sell off unsold seats at the eleventh hour.
(Reader's Digest April 2006) Macmillan English Dictionary的解釋：
the eleventh hour:
the last possible moment that you can do something,
especially prvent soemthing bad from happening.
因此應該有「期限到前的時刻」的意思，例如到最後購票期限前幾天或幾個小時。