Robinson's date for the text turns on on two matters: the irrelevance of the Muratorian Canon for the discussion, and the crucial relevance of Vis. 2.4.3. As Robinson notes, the argument for a second-century rests almost entirely upon the Muratorian Canon, which states that the text was written "recently," by a man named "Pastor," the brother of Pius, bishop of Rome from 140-154. Now, no question, the Shepherd is situated in Rome: that's clear from the text itself. But, as Robinson notes, the Canon is not generally considered to be a reliable source, and more recent scholarship seems inclined to push it from a late-second century date into the third and even fourth centuries. As Robinson notes, for no other text would chronological judgments rest almost entirely upon the canon's evidence, and thus it seems questionable to do so with the Shepherd. Moreover, Robinson is able to produce a compelling explanation for the text's likely error: the Latin title of the Shepherd is Liber Pastoris (Book of the Shepherd), and suggests that the author of the canon has simply made the mistake of assuming that this was the name of the author, i.e. Pastor.
More compelling is the data given by Vis. 2.4.3. Here, Hermas is having a vision of an elderly lady, who in Holmes' translation says to him
Therefore you will write two little books, and you will send one to Clement and one to Grapte. Then Clement will send it to the cities abroad, because that is his job. But Grapte will instruct the widows and the elders. But you yourself will read it to this city [i.e. Rome], along with the elders who preside over the church.There is a longstanding and likely correct supposition that the Clement referenced here (even if fictionally) is likely Clement of Rome, the putative author also of 1 Clement, and according to tradition the third bishop of Rome after Peter. It is sometimes supposed that this verse indicates (even if fictionally) that Clement was bishop at the time that Hermas wrote these words. Clement's episcopacy is typically dated to the 90s, although arguments have been made that would date its advent to the mid-80s. Robinson argues that this description of Clement's role suggests that Clement was more of a "foreign secretary" (his term) than a bishop proper, and thus that the text should predate his episcopacy. Given that the exact role of the "bishop" of Rome in these early decades remains unclear, I find myself generally unable to adjudicate between these two positions with confidence, and would be more inclined to say more generally that the Visions date to the last two or so decades of the first century. Other evidence adduced by Robinson points also in this direction. For instance, the reference to the elders who preside over the city (in Vis. 2.4.3, cited above) does seem to suggest an ecclesiology more like that which we know from the first rather than the second century.
We are left with the problem, identified by Osiek, that the text overall, while likely written by a single author, might well have been written over an extended period of time. I am perfectly happy to grant such a scenario, not least of all because of the sheer size of the work. But would this require us to move the completion of the text significantly into the second century, if at all? I'm not convinced, and thus am inclined to situate the Shepherd overall in the late first-century. Note that this would permit one to identify Hermas the author with the Hermas mentioned by Paul in Romans 16:14. Robinson however does not make this identification, and for good reason. Both are Roman Christians, in the first century (if the above hypothesis is deemed correct), named Hermas, but I'm not sure that suffices for an identification. This differs from the identification of the Clement of Vis. 2 with Clement of Rome, wherein the description of his role in the Shepherd matches closely the description of his role that we find in relation to 1 Clement. Absent such data, I am reluctant to think the Hermas who wrote the Shepherd is likely the Hermas mentioned by Paul. Certainly, the recurrent efforts to use Rom. 16:14 as a datum for dating the Shepherd need to be greeted with reserve.