This question asks about when until and by are interchangeable: in which situation such a prepositions "until" and"by" could be interchangeable?
The comments and answers there suggest that they aren't ever interchangable. However, I am wondering whether not until and by have the same meaning in the following examples. If they don't have the same meaning, what is the difference?
The report doesn't have to be ready until Friday.
The report has to be ready by Friday.
Those two mean the same thing: the deadline is Friday. There's a slight difference in tone, though.
The first wording (doesn't..until) has a more relaxed tone; it's almost suggesting that a little bit of procrastinating is okay. The second wording (has..by) suggests more of a sense of urgency.
So, let's say it's Tuesday, and I'm working on a report. A coworker asks me out to lunch. If I think I can go to lunch and still make my deadline, I might say:
Sure, let's go. This report doesn't need to be ready until Friday.
but if I think that going out to lunch now means staying late on Thursday, I might be more inclined to say:
No, sorry. This report needs to be ready by Friday. Maybe next week.
That said, this is a somewhat subtle nuance, and the two situations wouldn't necessarily require those respective wordings.
Similarly, let's say I'm the boss, and I'm assigning the report to a subordinate, who asks me, "When do you need this report?" If I answer:
The report doesn't need to be ready until Friday.
that implies I don't need it right away. I might say that on a Monday or Tuesday, but I wouldn't say that on Thursday. However, if I say:
The report has to be ready by Friday.
that implies a more urgent sense in the matter, and perhaps someone will be in hot water if the report is late.
The report doesn't have to be ready until Friday.
The report has to be ready by Friday.
The import of the first sentence is that there is some intervening time between now and the Friday deadline. It could be enough time, or even plenty of time. But some time. No need for panic, in any case.
I understand the second to mean that there's a Friday deadline. It might be a looming deadline or a distant deadline or a reasonable and very manageable deadline. We don't know from the text. It would depend on the tone of voice of the speaker.
P.S. But these differences aren't attributable to the choice of until over by. They're caused by the negative "doesn't have to be ready".
When is this report due?
-- We have until Friday.
-- We must complete it by Friday.
The answers are equivalent.
- The report does not have to be ready until Friday.
- The report has to be ready by Friday.
These two statements are not identical. In addition to the sentences using different words, the statements express different sentiments.
Both sentences indicate a period of time. In both sentences, the period of time begins no later than now and could begin much earlier. In the first sentence, the period ends on Friday. In the second sentence, the period ends before Friday.
This is similar to the distinction that we find when comparing "numbers between zero and one" and "numbers from zero through one". The majority of those two ranges may be identical, but the end points are defined differently. The number one is excluded from the first range of numbers but included in the second.
Let's assume that the report is finished at noon on Thursday. According to my first bulleted sentence, the report is ready early. According to the second, the report is ready on time.
"Early" and "on time" are not quite the same thing, even though neither is the same as "late".
Let me recast my sentences to more closely match your question:
- Not until Friday does the report need to be ready.
- By Friday, the report needs to be ready.
Similarities abound, but the contrasting elements are even more obvious. The sentence that uses "not until" also uses subject-auxiliary inversion. The "not" governs more than just the word "until" or the phrase "until Friday". Its impact is felt all the way down to the verb. The sentence that uses "by" also allows an optional comma, which would be an error if it were placed in the "not" sentence.
This may seem an insignificant difference in practical terms. If I know that it'll take me hours to compile the report, I won't notice the difference between 11:59:59.999 PM on Thursday and 12:00:00.001 AM on Friday.* However, it makes a significant difference in how the rest of the clause may be formed.
If the phrases in question were interchangeable, then the two following statements would both be correct:
✓ By Friday, the report needs to be ready.
✗ Not until Friday, the report needs to be ready.
_______________
* Yes, I understand that not every concept of Friday begins at midnight. For many people, the work day begins at 9:00 AM. For others, the business day begins at 6:00. Regardless, there will be some point in time that marks the beginning of the day in question.
