Source: H. Graetz, History of the Jews, volume II (Philadelphia: Jewish Publication Society of America, 1893) 430.
"Hadrian and Rufus's cruel measures were directed not against the survivors alone, but also against the dead. The heaps of dead [Jewish] bodies were not permitted to be interred, but the horrible sight was intended as a warning to the survivors that they should no longer dream of deliverance from the Roman yoke...It appears that a pious man desired to impress upon the survivors who had made peace with the Romans, and who lived in seclusion, the necessity of interring the corpses in the darkness of the night, even at the cost of their own happiness and peace. To this end he composed a book--the Book of Tobit--in which great weight is laid on the duty of secretly interring the bodies of those whom the tyrants doomed to disgrace; and at the same time it was hinted that the danger attending his duty would bring a rich reward. In evidence of this the case was cited of the pious Tobit, who after suffering many misfortunes as the result of his labor of love, was in the end rewarded with rich blessings. The contents of the Book of Tobit undeniably indicate that it was composed in the reign of Hadrian."
Most scholars these days do not date Tobit to the time of Hadrian, who ruled in the second century C.E. This is for a variety of reasons, one of which is that an Aramaic version of Tobit was found at Qumran, and Qumran was destroyed before the time of Hadrian. So obviously the book existed before then!
But Graetz was doing what he could with what he had at the time, in this case, the nineteenth century. And he noticed a parallel between something that happened in the time of Hadrian and the plot-line of Tobit. Many scholars today use the same sort of approach to date books--if a book has themes like a tyrannical madman and the Jews' desire to preserve their religion, then it probably originated in the time of Antiochus Epiphanes, who was a tyrannical madman trying to destroy the Jewish religion. Now that I see how Graetz fowled up while being so sure, I wonder how certain such an approach actually is.
On a related note, it amazes me how late some books of the apocrypha/deutero-canon may be. Many scholars date the Wisdom of Solomon to the first century C.E., since it reflects knowledge of Rome. Isn't that a little late to be part of the Old Testament?