What is the difference between the following sentences
At 6 o'clock I will have left. = The action of leaving will be complete by 6:00.
By 6 o'clock I will have left. = The action of leaving will be complete by 6:00.
Now to me they both mean the same, but, there is a difference of preposition, so there must be some difference between them, like there is when we use
a) At 6 o'clock I will leave. = The action of leaving will begin at 6:00.
b) By 6 o'clock I will leave. = The action of leaving will be before 6:00.
Please help me with this.
Thanks.