be to do sth.和be about to do sth.有什么区别?

e to do sth约定,注定,必定要做什么。
如:They were to meet at the usual place。
他们约好在老地方见．
be about to do sth 正要做什么
如：He was about to leave when the phone rang。
他正要离开就在这时电话响了．