Has this ever happened to you? Have you ever been sitting in a Starbucks, minding your own business, enjoying a good venti-sized vanilla bean frappacino, when suddenly some guy crashes his Dodge Ram truck through the window, rolls out the driver door dressed as Ronald McDonald, and proceeds to tell you that the Septuagint was written in the second century AD? You were probably thankful that you hadn't been hit, then after those sentiments of survival subsided you immediately asked the person a bit more about their position. You come to found out that they believe the copy of the Septuagint written before Jesus' time was merely the Law, and the rest of the Old Testament wasn't translated until the second century AD. Hence, when Matthew and the other Gospel writers quote the Old Testament in Greek, they were either inventing their own Greek verses, or the New Testament as a whole was written from the middle to late second century. You then tased him, not for what he said, but for the simple fact he's a lunatic crashing through buildings dressed as Ronald McDonald.
OK, maybe I've exaggerated this account just a little, but I did hear someone make the argument mentioned here.
The biggest thing we need to do is discuss some biblical history. When the books of the Old Testament were originally written, they were a mix of Hebrew and Aramaic texts. In the middle third century, under the funding of Ptolemy Philadelphus (then ruler of Egypt), it was decided to translate the Law into Greek, which had become the international language during the inter-testamental time. According to various stories, seventy-two Jewish scholars were selected, and finished translating the Law during the reign of Ptolemy Soter. It was perhaps the first major organized translation of scripture in history, similar to the work done by the King James Bible translators thousands of years later (and like the KJV, there's much mythology around its translating...but that's for another post).
The original Septuagint, as previously stated, wasn't the entire Old Testament, but rather was simply the five books of the Law. The question then comes: when were the other books finished? We have no solid evidence for the exact date that all the books of the Old Testament were finished. There are many signs, however, that much of it was done before the time of Christ, as seen by external evidence: the mid-second century Jewish historian Eupolemus mentions a Septuagint Books of Chronicles; the writer known as Aristeas quotes from the Septuagint Job; a footnote in an early Septuagint version of Esther suggests that it was in circulation before the end of the second century BC; the Septuagint Psalter is quoted in the apocryphal 1 Maccabees 7:17.
One of the biggest evidences we have that the Old Testament was completed by the time of Christ is found in the apocryphal work known as the Wisdom of Sirach. In the introduction, the author writes:
You are urged therefore to read with good will and attention, and to be indulgent in cases where, despite out diligent labor in translating, we may seem to have rendered some phrases imperfectly. For what was originally expressed in Hebrew does not have exactly the same sense when translated into another language. Not only this work, but even the law itself, the prophecies, and the rest of the books differ not a little as originally expressed. [RSV]While speaking of translating from the Hebrew into the Greek, the author makes mention of "the law itself, the prophecies, and the rest of the books." This is compounded with the fact that Philo and Josephus - two well known Jewish authors who lived during the lifetime of the early Christian church - quoted extensively from the Septuagint, and not just the Law. All historical signs point to evidence that the Septuagint, as in the complete Old Testament, was completed by the time of Christ. Most scholarly sources place its completion in the middle second century BC.
So, was the Septuagint written in the second century AD? Absolutely positively not - there is too much evidence to the contrary. So...where did this idea that the Septuagint was written in the second century AD come from? One can't just make this stuff up out of thin air. I personally believe the individual making this argument was confusing it with the various streams of Septuagint revisions, most of which happened about the second century AD. The ones most known about:
The revision by Aquila (early 100's AD). By this time, the Septuagint was becoming unpopular among Jewish circles, partially because of the rise of Christianity which heavily utilized the Septuagint. Aquila, a Jewish proselyte, attempted to make the first major revision to the Septuagint, and did so by translating from the Hebrew into Greek almost word for word. This made for a somewhat awkward rendition, but one that was popular among the Jews for the next 500 years or so. Today it is only known through fragments.
The revision by Theodotion (late 100's AD). Theodotion was a Jewish convert that relied heavily upon the original Septuagint. His version was heavily quoted by many Church Fathers (including Justin Martyr), and his version of Daniel was especially widely preferred by many over the Septuagint's version.
The revision by Symmachus (soon afterward). Symmachus was said by the writer Epiphanius to have simply been a Samaritan convert to Judaism, although Jerome and Eusebius claim he was an Ebionite. He sought to smooth Aquila's translation by using the original Septuagint and Theodotion's work as reference. His translation likewise only exists in fragments.
Perhaps a worthy final mention is that of the Church Father Origen (late second century, middle third century AD). Origen was a Christian who understood Hebrew, and saw differences between the Masoretic texts of his time and the Septuagint. He collected together what was probably the first interlinear Old Testament, as well as the first example of textual criticism. What he did was place side by side: the Hebrew; the Hebrew transliterated into Greek characters; Aquila's work; Symmachus' work; the Septuagint; and Theodotion's work. Origen even included notes and symbols that signified when the Septuagint added or left out specific parts of a verse (not unlike the use of italics and footnotes in today's translation). Unfortunately, this momentous work only exists in fragments today, although it helped to preserve examples of the Septuagint revisions done by the other three men.
So to repeat our question of the day: was the Septuagint written in the second century AD? As we've seen, some streams of it were, but the Old Testament translated into Greek was finished and widely available by the time of Christ. The writers of the New Testament were not making up Greek verses, nor is the Septuagint evidence that the New Testament was written in the second century AD. The contention made at the beginning of this post is, as stated before, simply a silly argument.
UPDATE, February 12, 2013: Another possible source of this confusion might be that the individual is using KJV-Only sources. Some KJV-Only advocates try to teach that the Septuagint comes from a later date, even after the time of Christ. One such KJV-Only advocate writes: "People who believe that there was a Septuagint before the time of Christ are living in a dream world." (pg. 50; Peter Ruckman, The Christian's Handbook of Manuscript Evidence, 1976).