The other day I came across this passage in a nineteenth-century Vietnamese geographical text, the Lê triều cống pháp 黎朝貢法 (A.53, pages 1b-2a). This is a text which is supposed to be a revised version of a geographical work which the fifteenth-century scholar-official, Nguyễn Trãi, first drafted. What struck me was a passage which this text included by the late-eighteenth-century scholar, Ngô Thì Sĩ. This text quotes Ngô Thì Sĩ as having written the following:
自寅會生人之後，人種既繁，必有出類而君長之。起自人皇，以至神農，蓋世論十紀中之第九禪通也，悠悠南服，有山川必有人物，況神農時所云，南撫交趾，固已別成一國，豈應至神農四世為疏仡紀，而涇陽王為首出之帝王。
“From the time that humans were created in the third phase, various sorts of people proliferated. There had to be a superior type emerge who would rule over others. Starting from Renhuang and continuing to Shennong, or what people at the time termed the Shantong [era], the ninth of the ten eras, the remote southern submitted [zone] had mountains and rivers, and must have had personages [of worth]. What is more, during the time of Shennong, as it was said, ‘to the south [he] assuaged Jiaozhi.’ Therefore, it had already become a separate kingdom. Why would it have been necessary to wait until the time of a fourth-generation descendent of Shennong in the Shuyi era for King Kinh Dương to emerge as the first emperor?”
The first thing which surprised me about this passage was that Ngô Thì Sĩ employed terms to refer to periods of time which I was unfamiliar with. I had never heard of “the third phase,” “the Shantong era,” or the “Shuyi era.” As it turns out, this reference to “phases” appears to come from the opening passage of the novel, the Journey to the West (Xiyou ji). This is how that work begins:

蓋聞天地之數，有十二萬九千六百歲為一元。將一元分為十二會，乃子、丑、寅、
卯、辰、巳、午、未、申、酉、戌、亥之十二支也。每會該一萬八百歲。

“In the arithmetic of the universe, 129,600 years make one cycle. Each cycle can be divided into twelve phases: I, II, III, IV, V, VI, VII, VIII, IX, X, XI and XII, the twelve branches. Each phase lasts 10,800 years.” [I found this translation online: Adapted from the WJF Jenner translation (Beijing, 1955) by Collinson Fair. Copyright 2005, Silk Pagoda.]
In any case, this is the only place where I can find these terms.
As for the Shantong and Shuyi eras, these terms do not appear in the official dynastic histories. However, this information is apparently in Liu Shu’s 劉恕 (1032-1078) Zizhi tongjian waiji 資治通鑒外記, where the period of what we today consider to be the mythological history of ancient China is clearly divided into ten eras. How Liu knew the information he did, thousands of years after the fact, is anyone’s guess. . .
Nonetheless, what is interesting here is that Ngô Thì Sĩ is arguing that the first “emperor” to emerge in “the south” must have done so (or could easily have done so) earlier than Vietnamese histories claimed. Ngô Sĩ Liên’s fifteenth-century Đại Việt sử ký toàn thư 大越史記全書 had recorded that the fourth-generation descendent of Shennong, King Kinh Dương, was granted the authority to rule over a kingdom in the south by his father, Diming, a figure who is also not mentioned in official Chinese dynastic histories but who appears in the eleventh-century Zizhi tongjian waiji. Further, King Kinh Dương is not referred to in that or any other work that I know of as an “emperor.”
Ngô Thì Sĩ makes this point by stating that during the time of Shennong it was said that “to the south [he] assuaged Jiaozhi” (南撫交趾), and that this must mean that Jiaozhi was already a kingdom, and this implies that it must have had an “emperor.” “Jiaozhi” is the oldest term for the far south of the world as it was known to the ancient Chinese. It is not clear if it referred to the area of what is today Vietnam, but it eventually came to be associated with that land in the minds of both Chinese and Vietnamese. The problem, however, is that this expression, “to the south [he] assuaged Jiaozhi,” is recorded in such works as Sima Qian’s Shiji 史記 in reference to the emperor Shun, who supposedly lived long after Shennong.
Ngô Thì Sĩ surely would have known that this expression referred to the activities of Shun, and he surely would have known that no one had ever referred to King Kinh Dương as an emperor. So what was he up to here? It seems obvious that he was trying to push the origins of an imperial enterprise in the region further back into the past. It is interesting to see that he was relying on what we could call vernacular or non-mainstream sources to do this. That is what Ngô Thì Sĩ had relied on in writing this portion of his own history as well. What was the need, however, to push the origins of an imperial enterprise back four generations? And given how famous the statement, “to the south [he] assuaged Jiaozhi,” is in reference to Shun, who would have believed him?