Learn Hardcore Greek: Μέχρι τις δώδεκα θα έχω τελειώσει τη δουλειά. - By twelve o’clock I will have finished the work.

Why? Because the main verb is a completed action: θα έχω τελειώσει = I will have finished. So the idea is that the work will be completed no later than twelve.

Μέχρι can mean until in some contexts.
But with a completed event like this, it often works like by.

So:

Μέχρι τις δώδεκα θα έχω τελειώσει τη δουλειά.
= By twelve, I will have finished the work.

Why is it τις δώδεκα and not just δώδεκα?

Greek often uses the definite article with clock times:

στις δώδεκα = at twelve
μέχρι τις δώδεκα = by/until twelve

So τις is not unusual here; it is the normal way to refer to a specific hour.

Grammatically, τις δώδεκα is in the accusative, because the preposition μέχρι takes that form here.

You may sometimes hear μέχρι δώδεκα in casual speech, but μέχρι τις δώδεκα is very standard and natural.

Why do we use θα έχω τελειώσει instead of θα τελειώσω?