# Using simple future and future perfect

What is the difference between the following sentences

1. At 6 o'clock I will have left. = The action of leaving will be complete by 6:00.

2. 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.

Thanks.

At 6 o'clock I will leave/have left.

This means: I will leave/have left at exactly 6 o'clock.

By 6 o'clock I will leave/have left.

This means: I will leave/have left at 6 o'clock at the latest, but probably before that. I'm not sure when exactly I will leave. But I will have left until 6.

• So does 'I will have left at 6 o'clock' mean I will not leave before 6 o'clock? Am I right? Jul 26, 2016 at 14:52
• You wouldn't say that. "will have left" indicates the process of leaving, which is not a point in time. Jul 27, 2016 at 2:16
• @shikha ji: "I will have left by x" means you will be gone at x because you left some time before. Either seconds or hours before x, but at x you are gone. Jul 27, 2016 at 20:06
• @someasw Can't "I will have left by x" mean I will leave at x? because by definition, 'by' means either before or at. So is it not possible to leave at x? This is confusing. Jul 28, 2016 at 2:07
• @shikha ji: Yes, "I will have left by x" might also mean "I will leave at x". But you would rather use it to express that you will leave some time BEFORE x (hence using "by"). If you want to express that you will leave exactly at x, you say "I will leave at x". Jul 28, 2016 at 13:38