I will have left at 8 o'clock.
I will have left by 8 o'clock.
Both are grammatically correct. You must use 1st one. I will have left at 8 o'clock.
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Both are grammatically correct. You must use 1st one.
I will have left at 8 o'clock.
anonymousI will have left by 8 o'clock.
This one is preferred.
The future perfect is often used with "by + a time", "before + a time", or "in + a time".
I will have left here in a week.
I will have left here before John arrives.
I will have left here by the end of the week.