Once again you fail to furnish us with one single quote from Barnabas describing a future millennium. That is because it does not exist. He was an Amil. The fact you have had to run to Justin Martyr for support demonstrates your lack of evidence.
One of the most thorough researches on the Epistle of Barnabas comes from Premillenarian historian D. H. Kromminga in his book Millennium in the Church. In it he gives lengthy consideration to the eschatological position of Barnabas. Kromminga emphatically concludes that he was not a Chiliast but was in fact an Amillennialist. He acknowledges: "Now, it would seem, that this argumentation would land Barnabas right in the lap of the millennium as a final period of this world’s history. He is perfectly aware of this and does not at all shun this consequence. However, he explains the statement that God rested on the seventh day, as follows: “this meaneth, when His Son, coming, shall destroy the time (of the wicked man) and judge the ungodly and change the sun and the moon and the stars, then shall He truly rest on the seventh day.” And the fact should not escape our attention, that in this chapter Barnabas links the notion of the rest with both the seventh and the eighth day."
He continues: "He seems to be of the opinion that there will be a seventh world period all right, but that period will be identical with the perfection of the eternal state. There can be no doubt about the identity of his seventh and his eighth day. The day of rest that is coming is one and the same day, viewed from 2 different aspects. From the viewpoint of continuity the great world–sabbath is a seventh day; but from the viewpoint of discontinuity it is the eighth, beyond and outside the present world–week. The future state is the last reckoning from creation; it is new, because of sin and redemption. This is the simplest meaning which I can discover in Barnabas’s words; but this is plain and pure Amillennialism."
Whilst Barnabas was not a Chiliast you can easily see how his 6 day/6,000 years theory opened up the door to the conclusion that the 7th day will also be 1,000 years long. Even though most of the early writers considered the 7th day as eternal, the whole basis of the idea of a millennial week was fraught will many factual discrepancies, human speculations and theological contradictions. A study of the early fathers will see that this faulty concept inevitably led to some embracing Chiliasm.
Stanley J. Grenz writes in The Millennial Maze: “The creation-day world-age theory that Justin and others employed did not necessarily lead to the materially oriented premillennialism of Irenaeus. This is exemplified by a work that probably predates the early apologist … the Epistle of Barnabas.”
Alan Patrick Boyd says in his master’s thesis presented to Dallas Theological Seminary (1977) “A Dispensational Premillennial Analysis of the Eschatology of the Post-Apostolic Fathers” and particularly his study of the Epistle of Barnabas: “In the light of the argument of the passage, can one conclude that the author (Barnabas) was a premillennarian? Probably not, for the following reasons. First of all, one must realize that a belief in six millennia of world history in no way obligates one to posit a seventh millennium in world history. In other words, the most modern scholarship can do is to assume that the Seventh Day, in the writer's thought, is a millennium since there is no prima facie evidence for it. One must not assume the part (six millennia) for the whole (seven millennia). Secondly, the concept of ‘rest from creation’ is given within the chronological framework of the second advent (15.5) and the beginning of the Eighth Day (15.8)” (p. 104, 105)
He adds: “Therefore, the Eighth Day can be said to begin at the Second Advent. In other words, the Seventh Day is eschatologically the beginning of the Eighth Day. Therefore, there is no interval (millennial or otherwise) between the Seventh and Eighth Days” (p. 105).
He continues: “The whole point of the chapter is that the Eighth Day is the acceptable Sabbath. In the light of this, the seventh Day plays no appreciable role. If the Seventh Day were a millennium that millennium is insignificant” (p. 105).
He explains: “In the light of this, it seems best to understand the Eighth Day as eternity, and since the Seventh Day is synonymous with the Eighth, the Seventh Day would also be eternity” (p. 105, 106).
He states: “In the light of the unity of the Seventh and Eighth Days, if a millennium were to exist, it would only be the threshold to, but within, eternity. It would be no interval between the present age and the eternal state (BOYD'S FOOTNOTE- this seems to be similar to Justin Martyr’s thought)” (p. 106).
He says: “In conclusion then, it seems best to conclude that Barnabas was not a premillennarian. The existence of an eschatological Millennium in the author’s thought can only be an assumption of modern scholarship” (p. 106).
He concludes: “…the sanctification of the Seventh Day, is not presently being fulfilled, but will be fulfilled when: 1) Christians are justified; 2) Christians are resurrected and rule the earth, 3) there is no more sin; 4) there is a new universe; 5) God causes everything to rest; and 6) God makes the beginning of the Eighth Day, i. e. – when God begins another world” (pp. 103-104).