Understanding deadlines when using UP TO, BEFORE, BY & UNTIL

Christmas is just around the corner and there are deadlines to be met.

Your work and Santa´s presents have to be ready before the 25th of December, by the 24th of December to be exact. At midnight of the 25th, he should have finished his deliveries and you should be relaxing with your family having celebrated a pleasant day without stress.

That is if you managed to meet your deadlines.

In English, deadlines are often established using a variety of expressions which include BY, UNTIL, BEFORE and UP TO, among others. Brazilian Portuguese relies on the word até.

The words BEFORE and UP TO infer a deadline by stating the day or date.

The work must be ready BEFORE Friday infers that the deadline is Friday but that the work should be completed on Thursday ready for Friday.
I need the number of hours worked UP TO (and including) the last day of the month BEFORE Friday. Here we see that the deadline inferred in calculating the number of hours worked is related to the range of dates from the first day (not defined here) to the last day being the end of the month.  Friday establishes a deadline for delivery as in the previous sentence.

BY and UNTIL are used to stipulate a specific deadline.

The work must be completed BY Friday. (This form establishes a priority and date that cannot be passed.)

You have UNTIL Friday to complete the work. (This form also specifies a date that cannot be passed, but greatly alters the concept of priorities). It is assumed here that there is enough time before the deadline to be able to prioritize other tasks as long as the deadline is respected.

What is most important here is to recognize the difference in the structures used. They ARE NOT interchangeable.

Contrast the difference in meaning by using  BY or UNTIL  in the following example.
I will be in the office BY 15:00 (3pm). This infers that the person is outside the office and will return to the office some time before, but no later than 15:00 (3pm).
I will be in the office UNTIL 15:00 (3pm). This, on the other hand, infers that the person is in the office and will leave the office at 15:00 (3pm).

As mentioned earlier, Brazilian Portuguese relies on the word até. This word translates primarily to UNTIL while BY is understood to mean até as well. Because of there being only a single word in Portuguese to mean both BY and UNTIL, there is often a lot of confusion as to which of these two words they should use. Whereas in English we might clearly understand what is implied through the use of one or the other, Brazilians will not always, "get" this difference.

To make things worse, the word até is also very close to the English preposition AT. This results in a kind of double translation where AT is read as até then translated into BY which ends up being used in the following way - "I'll meet you BY 15:00 (3pm)" when in fact the phrase should be "I'll meet you AT 15:00 (3pm)" 

Once you understand these difficulties it is up to you to make sure you communicate exactly what you mean.
"The last day or The deadline for delivery is day XXXX or date XXXX" might be easier to understand than "The XXXX has to be delivered BY XXXX".

While this situation may not always seem to be critical, the right choice of words reduces the doubts and head scratching that comes about through the misuse of similar meaning terms. For the native English speaker this is also a chance to reflect on how you communicate with other non-native users of English.