Dating of Pentateuchal Sources
Having been engaged in the question of the dating of the sources of the Pentateuch for some time, there is one criterium that I have encountered repeatedly. This is the idea that sources can be dated by identified by finding time periods in which they would have answered the needs of the community. This has been done by a number of different scholars. Van Seters dates J to the exile in part because it would have served a community in exile. Levine argues that P should be considered postexilic because it would have met the needs of those returning from Babylon. Examples could be multiplied ad naseum.
This seems to me, however, to be a very poor criterium. Sure, the biblical text met the needs of the exilic community and the postexilic community, but they have also met the needs of any number of time periods since then. This is one of the dynamic aspects of the Bible and one of the reason it continues to be used as Scripture. If the text had not continually met the needs of the community, it would not continue to be used.
Identifying the Sitz im Leben of the biblical text is an important step in interpretation. But it is a step that should be taken after a text has been dated on other grounds. If we can date J to the exilic period (and I think Van Seters has made that case in other ways), then and only then is it valid to ask the function it would have served in the exile. To try to date the text on the basis of whether it answered questions is much too subjective, since it answered questions in many time periods.
The best criteria are historical, linguistic, and comparative. Historical references in the text give us a general time period. Linguistic data will provide us with relative dating, although we still do not have a complete idea of the development of the language through the centuries. And comparative studies with other books provide us with a relative date. This, for instance, is what Avi Hurvitz has done in showing that P is relatively earlier than Ezekiel, even though it does not provide us with exact dating. None of these is a fool proof means of dating of course, but they are better than trying to pinpoint the date on the basis of a supposed Sitz im Leben.