As I noted in DH-33, although there are various schools of thought, biblical scholars can’t agree on how many supposed sources contributed to the Pentateuch. Unfortunately, things are even more complicated. There is also no agreement concerning the date of J among those scholars who agree that there is in fact a J-source. (Here is a summary of views of this who dispute the nature or existence of J in various ways; and more fully Dozeman, Thomas B; Schmid, Konrad A Farewell to the Yahwist? SBL, 2006)
The following are different dates for J suggested by people who accept one form of the Documentary Hypothesis in one form or another.
10th century BC (von Rad, 1938)
8th century BC (Friedman)
7th century BC (Schmid, The So-called Jahwist, 1976)
5th century BC (Exilic or Post-exilic) (Van Seters, Abraham in History, 1975)
3rd-2rd century BC (Thompson)
Now, imagine that we had a literary theory and method that permitted reasonable arguments from informed scholars proposing dates for Shakespeare ranging from AD 1300 to 1800. No one would take such a method seriously. In ordinary historical studies, a methodology with such a lack of methodological and theoretical control that it permits hypothetical dates for a text ranging over half a millennium would be considered a failure and be rejected. If Documentarians encountered texts in the Pentateuch that proposed five different dates for a patriarch, they would insist that there were five different sources, not universal consensus of a single source.
Part of the problem is, of course, a lack of sufficient data to resolve the issue. But, to that extent, the proper response should be to admit we don’t know, and are unable to resolve the debate. It is certainly not grounds for claiming the success of a method that has achieved scholarly consensus. The weaknesses of the method and lack of data necessary to resolve the question are not excuses for further speculation. It is rather a mandate for rejecting further speculation.