In the Afterword to his Classifying the Zhuangzi Chapters, Liu Xiaogan takes up the problem of dating the Laozi (Daodejing). Once again, he concludes, based on language and style, that the traditional view that it was written by an elder contemporary of Confucius (551–479 B.C.E.) is most likely. I find his arguments convincing (in the context of my own lack of expertise), but the question is far from entirely resolved. I would note also that Liu tends to prejudice traditional views and considers them best until proven otherwise. Personally, I do not share that trust.
For my part, after no analysis, I have suggested that the Laozi was written (or perhaps finally compiled and codified) after Zhuangzi wrote the Inner Chapters and before the Outer and Miscellaneous Chapters, which frequently quote it. Zhuangzi, however, does not quote it at all, even though he frequently makes use of the reputed author, Laozi (Lao Dan) in his stories. I have also suggested, though again without any real evidence, that Zhuangzi might have even made Laozi up as he did so many other characters. In any case, he is not loath to quote other philosophers — Confucius, Huizi, Gongsun Long, and Song Rongzi come to mind — so why would he not quote Laozi in whose mouth he puts so many of his own words? That he is aware of the writings of these and other philosophers, moreover, would suggest that if there were a Laozi at the time of his writing, he would have probably been familiar with it.
This is even more curious when we consider how Zhuangzi's writing has traditionally been seen as an elaboration and amplification of the Laozi. He is not much of a disciple who completely ignores the words of his master. But then, I do not think this view is correct; though they have much in common, Zhuangzi's philosophy is something else altogether, and though he certainly inherited much from others working on similar paths, his thought is in many ways unique. To my thinking, if one follows a path inspired by the philosophy of Zhuangzi, then the Laozi is only secondarily helpful, not the other way around.
One reason I am happy to break this supposed link between Zhuangzi and the Laozi is because the latter is largely thought to contain metaphysical statements which are then projected onto Zhuangzi. Hansen, in the previously discussed article dismissive of a Zhuangzian mysticism, makes oblique reference to this link as evidence of a traditional imposition of a mysticism full of metaphysical content. Whether the Laozi does or does not contain such statements I will not venture to say, but I will say that Zhuangzi does not.
You can check out Scott's writings on Zhuangzi here.