You cite something in your excellent Shortening of the Days thread that reminds me of something from the Letter of James and I thought I'd comment on it here.
Talmud BM 85b:
Mishnah Ber. 5:2 links praying for rain with the resurrection of the dead and crops:Elijah used to frequent Rabbi's academy. One day — it was New Moon — he was waiting for him, but he failed to come. Said he to him [the next day]: "Why didst thou delay?" He replied: "[I had to wait] until I awoke Abraham, washed his hands, and he prayed and I put him to rest again; likewise to Isaac and Jacob." "But why not awake them together?" "I feared that they would wax strong in prayer and bring the Messiah before his time."
[We] mention the request for rain in [the blessing of] Techiyat HaMeitim [Resurrection of the Dead]; and [we] ask for rain [by adding the phrase Vetein Tal Umatar Livrakhah] in Birkat HaShanim [Blessing of the Year's crops].
https://www.sefaria.org/Mishnah_Berakhot.5.2?lang=bi
James 5:7-18:
So here we have grounds for more than just assuming that James and his readers were aware of the implication of messianism and the resurrection of the dead with respect to the last days in the Letter of James. And these similarities are in keeping with the idea that Jewish Christians were a faction of Fourth Philosophy, who Josephus says, aside from their rejection of "the customs of our fathers" (Ant. 18.1.1), "agree in all other things with the Pharisaic notions" (Ant. 18.1.6).Be patient, then, brothers and sisters, until the Lord’s coming. See how the farmer waits for the land to yield its valuable crop, patiently waiting for the autumn and spring rains. You too, be patient and stand firm, because the Lord’s coming is near ...
Elijah was a human being, even as we are. He prayed earnestly that it would not rain, and it did not rain on the land for three and a half years. Again he prayed, and the heavens gave rain, and the earth produced its crops.
Darlack has an abstract of his thesis "Pray for Reign: The Eschatological Elijah in James 5:17-18" here, which discusses these kinds of similarities:
James' reference to three and half years of tribulation in 5:17 possibly has additional Danielic messianic resonance, as a comment on the bible hub suggests.James uses the prophet Elijah as an example of righteous prayer. This thesis explores the possibility that James may have intended his readers to recognize both historical and eschatological imagery associated with the biblical prophet. First, it shows that in early Jewish literature the eschatological and historical Elijah traditions were not held in isolation of each other. Imagery from descriptions of Elijah’s eschatological return is used to describe the pre-ascension ministry of the prophet, while the eschatological mission of the prophet is described using elements of the historical narrative. Second, the thesis demonstrates that James’ prescript “to the twelve tribes of the Dispersion,” sets a tone of inaugurated and yet-to-be-consumated eschatology, and that the mention of Elijah helps form an eschatological inclusio that frames the letter. Third, the New Testament use of Elijah’s drought outside of James is explored showing again that elements from Elijah’s drought in 1 Kings were used in eschatological contexts, and that Elijah’s three and a half year drought, as mentioned by James, is used to illustrate a period of judgment for the sake of effecting repentance in these contexts. Fourth and finally, the images of rain and drought are viewed through an eschatological lens, revealing their role as covenant blessing and curse, and eschatological judgment and restoration. It is concluded that James’ readers could have recognized the eschatological implications of using Elijah as an example of faithful, righteous prayer, and that James assigns his readers a role similar to that of the eschatological prophet. They are called to endure in the midst of eschatological trials and to effect repentance before the arrival of the soon-coming King.
http://www.oldinthenew.org/?p=143
So for these reasons (and the fact that it may have been known to Hegesippus, appears to reference Paul and is in a Christian canon) the Letter of James (including the references to the coming of the Lord in 5:7-8) seems messianic to me.... it was probably the time handed down by tradition, being taken by the Jews as a symbol of times of tribulation (cf. Daniel 7:25; Daniel 12:7; Revelation 11:2).
http://biblehub.com/commentaries/james/5-17.htm