The cutting room

Kronk’s record for the 776 AD comet runs as follows:

776. The Chinese text Ku chin t’u shu chi ch’eng (1726) is the sole source of information for this comet. It says a comet appeared at Hu-Kua [α, β, γ, and δ Delphini] on 776 January 11. The date and location indicate it was in the morning sky, implying a UT of January 10.9. The Chinese added that the comet “trespassed” Huan-Chê [60, SAO 102553, and SAO 102564 in Herculis, and 37 Ophiuchi], but remained visible for only 20 days.
FULL MOON: January 10
SOURCES: Ku chin t’u shu chi ch’eng (1726), p. 92; I. Hasegawa, “Catalogue of Ancient and Naked-Eye Objects”, Vistas in Astronomy 24 (1980), 59-102; 73, 92 #437*.

Hasegawa had translated the source:

On a i-hai day in the 12th month of the tenth year of the Ta-Li reign-period a comet appeared at Hu Kua (α, β, ν Del). It was several feet in length, after 20 days it disappeared. It trespassed Huan-che (near α Her).

"i-hai" is in the Shang-era sexagesimal numeric system; the yi-hai number from the new moon is indeed 12. This date is off only by one or two days to the “white vapor” noted in more-contemporary Chinese sources for 12 January that year.

Also to be noted, is a record for 767 AD. I'll follow its quote to John Williams. I'll correct it slightly.

A.D. 767. January 12. In the reign of Tae Tsung, the 1st year of the epoch Ta Leih, the 12th moon, day Ke Hae, there was a comet in Kwa Chaou. It was about a cubit in length. After 20 days it disappeared. It passed over Hwan Chay.
Emperor Tae Tsung [18 May 762 – 23 May 779]; epoch Ta Li, 766-79; ... 12th moon, day Ke Hae, January 12, 767.
Kwa Chaou, α, β, γ, &c. Delphini.
Hwan Chay, ε, ι, &c. Ophiuchi.

Shiji continuatus; Mă Duānlín's encyclopaedia Wénxiàn Tōngkǎo

"Ke Hae" would in modern Mandarin be ji-hai which is 36. A bit much for a month, but...

I had, last week, already decided that the 776 CE record in Ku chin t’u shu chi ch’eng is an eighteenth-century interpretation of its own sources, allied to those which record the “white vapor”. Recently I found J. Chapman, D.L. Neuhauser, R. Neuhauser, M. Csikszentmihalyi, “A review of East Asian reports of aurorae and comets circa AD 775” / “Chinese aurorae in the AD 770s”, Astron. Nachr. 336.6 (2015), 530-44; 540-2 #5.2.

It may be that the Ku chin - ah hell, the Gujin tushu jicheng - recorded a comet immediately preceding the vapor which the earlier sources describe. More likely, the phenomena are the same, in which case the modern scholar should prefer those sources closer to the event. Such a reader would interpret the Gujin record of that 776 event as misinterpretation.

But I figured that Chapman has already did the work for us. I am personally incompetent in Tang-era Chinese, and in Qin-era for that matter. If someone else wants to figure out how Williams calculated his date for 767, and what the Gujin was on about for 776; be my guest.