- 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.