By 8 o'clock ,at 8o'clock 和on 8o'clock什么区别

问题描述:

By 8 o'clock ,at 8o'clock 和on 8o'clock什么区别

应该是:
by 8 o'clock 是在8点附近,一小段时间范围内
at 8 o'clock 是在8点,只是一个时间点
on 8 o'clock 应该是不对的,没有这种用法