[Grammar] “By” certain time VS “before” certain time

Are these two sentences interchangeable?
Don't worry - we'll finish the project BY Christmas.
Don't worry - we'll finish the project BEFORE Christmas.
Or they are slightly different?
The same:
Yes sir, you must leave BY 8 o'clock a.m.
Yes sir, you must leave BEFORE 8 o'clock a.m.

Assuming Christmas refers to 25 December, 'by Christmas' and 'before Christmas' mean 'on 25 December at the latest' and 'on 24 December at the latest' respectively.
'By 8 am' and 'before 8 am' mean 'at 8 am at the latest' and 'at 7:59 am at the latest' respectively.

I'd say "we'll have finished the project by Christmas". Is it OK?

Boris Tatarenko said:

I'd say "we'll have finished the project by Christmas". Is it OK?

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It's correct, as is the original.

northpath said:

8 o'clock a.m.

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I would write either '8 am' or '8 o'clock in the morning'.
See http://www.eslcafe.com/grammar/telling_time04.html

emsr2d2 said:

It's correct, as is the original.

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How is it even possible? Is there at least a slight difference between them?

In this context, there's no difference between

'Don't worry — we'll finish the project by Christmas'

and

'Don't worry — we'll have finished the project by Christmas'.

I found a difference in the following context:

1. 'We'll finish the project when you arrive.' ── We'll finish it immediately after you arrive.
2. 'We'll have finished the project when you arrive'. ── We'll finish it before you arrive.

Different prepositions have different uses.

by can be used to talk about time limits, such as deadlines. With this usage, the sentence means any time between now and a specified point in the future at the latest (Christmas).

before is simpler. It's really just a way to sequence events along a timeline. It means not after Christmas, i.e. finish the project first, then Christmas.

You can see that there is a clear overlap of temporal meaning (they both mean before Christmas), but the better choice of preposition, in this example, will depend on the intended meaning of the speaker. I'd say it's most likely to be by as projects often come with deadlines.

Don't think about the meaning of prepositions. Think about the use, and how that use can help speakers say what they mean.