I've been thinking about the date of Hebrews. Hebrews is a notoriously difficult text to work with for those of us interested in what Lonergan calls "basic history": "basic" in the sense of dealing with foundational matters, such as chronology. Out of the twenty-seven books of the NT it is the one text that does not have even a putative author, and the old saw (going back to Origen) that only God knows who wrote the letter is probably not far from the mark. We can however venture some guesses as to when it was written.
There are, I think, four crucial data when thinking about this matter: the apparent references to Hebrews in 1 Clement (notably ch. 36, but upwards of fifty references have been identified at different times by different scholars); Heb. 13:23; the traditional title, "To the Hebrews"; and Heb. 10:2. I have organized the material in this order, as it allows us to consistently narrow down the likely range for the epistle's date.
The Clementine references to Hebrews as well as Hebrews 13:23 together point to a probable date before c. 90ish. 1 Clement is typically dated to the mid-90s, although there have been arguments that would place it around 70 (the strongest argument for which is probably 40-41, which could be read as indicating that the Jerusalem temple is yet standing); for our purposes we can go with the majority date on this matter. Certainly, if there is anything to the traditional attribution to Clement of Rome, 1 Clement can't be much if at all later than c. 100, when Lightfoot dated the end of Clement's bishopric (allowing that, whatever it meant to be "bishop" in that era was probably unlike what the term later came to indicate, this probably does give us a good sense of when Clement flourished). Hebrews 13:23 also points to a similar terminus ante quem: if the Timothy referenced in this passage is the only other Timothy known from the first Christian century, and there's frankly neither good reason to think otherwise nor good reason to multiply entities here, then it is difficult to imagine that a man active in Christian ministry by probably the late-40s and certainly by the 50s (cf. Acts and the Pauline epistles) would still be flourishing much later than the 90s. Both together suggest a probable maximal limit for Hebrews at the 90s. As for terminus post quem, the reference to Timothy gives us perhaps a lower window of c. 50, as we have no clear evidence of his activity as a Christian leader prior to the Jerusalem council, although a date in the 40s probably couldn't be ruled out. Tentatively though we can suggest that Hebrews was most likely written between 50 and 90 of our era.
The traditional title, Πρὸς Έβραίους (Pros Hebraious), tentatively points us towards a date in the lower half of the 50-90 frame. Now, of course, the title might very likely not be original to the epistle, although it is not clear that it is secondary. There is, to the best of my knowledge, no positive evidence that the letter ever circulated absent this phrase at the beginning. At the very least, if it is secondary, it entered the textual tradition very early, and it cannot be assumed that such addition was made in complete ignorance of the historical situation of the letter. As such we can tentatively treat the title as evidence for the situation in which the text originated. When doing so it is worth noting that we know of a group of Christians called the Hebraioi (the plural nominative of Hebraios, whilst Hebraious is the plural accusative of the same word): according to Acts, it was one of the sub-groups of the Jerusalem church. Now, the Jerusalem church appears to have dissolved in the 60s, perhaps around the beginning of the Jewish War, and although there is evidence that it was reconstituted in Pella and then in Jerusalem after the revolt there is no real evidence that there was ever again a group within the church that went by the name Hebraioi. Thus, again tentatively (note the repeated italics), I suggest that the epistle was written to this group before its dissolution in the 60s.
On its own the argument from the traditional title is not particularly strong, however. The exact relationship of the letter to the title is too unclear, and even if granted as evidence for the question of date we cannot with a high degree of confidence rule out that the Hebraious of the title do not have a referent other than the Hebraioi of Acts. It is really more suggestive than anything. Heb. 10:2 however likewise points to a pre-70 situation. More than one scholar has argued that the reasoning of Hebrews regarding the temple makes more sense before rather than after 70. If Hebrews' argument is to show that the temple establishment is obsolete then what better way to urge this point than to observe that it no longer operates? This is exactly what the Epistle of Barnabas, a clear post-70 text, will do. I think that in and of itself this argument has some weight, but is not decisive: it turns too much on what we think the author would have done under certain circumstances. 10:2 however is different. It reads, in the NRSV, as follows: "Otherwise, would they [i.e. the temple sacrifices] not have ceased being offered, since the worshipers, cleansed once for all, would no longer have any consciousness of sin?" One could I suppose argue that Hebrews means something like "would they not have ceased being offered before Jesus came?", but that seems only necessary if we assume a post-70 date. The far more straight-forward reading is that there is an unspoken minor, which makes the logic of the passage proceed as follows: If the sacrifices could cleanse once and for all they would have ceased; they have not ceased; therefore they do not cleanse once and for all. Indeed, writing after 70 the author would have faced the possible rejoinder that they have in fact ceased; that he (possibly she) evinces no concern for this rejoinder seems to make more sense pre-70.
Could Hebrews have been written between 70 and 90? Yes, no question, that remains a viable possibility; nothing definitively rules it out. On the balance of the evidence something between 50 and 70, and perhaps not too close to 70 due to the dissolution of the Jerusalem church early in the Jewish War, seems to make more sense. As for who wrote this letter, I wouldn't even hazard a guess, although we might be able to say a bit more about its situation. I would probably rule out a Roman origin, as the reference in 13:24 to "those from Italy" sending greetings seems to suggest that the writer is not writing in Italy: if she or he is with people who came from Italy then she or he is probably not in Italy. I would suggest that the writer is someone who is at least familiar to or with Pauline circles, due to his companionship with Timothy (cf. again 13:23). Someone not in Rome at the time of composition, active c. 60, and with Pauline connections: that leaves virtually all the usual suspects and even some unusual ones (such as Apollos, Barnabas, Clement himself, Luke, Priscilla, Aquila, Silas) still in play, without any clear way to adjudicate between them, and as such any more precise statement than that and we're probably going beyond what the evidence will allow us to say. We might add that of these Barnabas, Silas, and Luke have known connections with both the Jerusalem church and with Timothy, thus perhaps shifting the balance of probabilities towards such figures (if indeed the author was someone that we know about from broader tradition, which cannot be taken as a given), but truthfully any effort to be too exact is going to be problematic.