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If you are a Stack Overflow user you may already have read Jon Skeet's excellent answer that discusses a time zone change at the end of 1927 in Shangai, when the clocks went back for 5 minutes and 52 seconds.
While I understand that time changes are somewhat common, especially with the advent of railroads, I can't find any information on the political or administrative reasons that lead to this specific change.
Help?
1927 saw Shanghai change control from local warlords to the Kuomintang Nationalist Government, who then purged the Chinese Communist Party on April 12 and then declared Shanghai to be a municipality in the Republic of China.
Presumably the time change to GMT+8 from a more local mean time was a combination of desire for modernity and to show that Shanghai was part of one China.
I'm actually deeply suspicious about the data used on TimeAndDate for historical times, and I suspect things were a lot more messy on the ground in China in late 19th to mid-20th century, even with the growth of telegraph etc.
I did, however, poke around some postings in Chinese about Jon Skeet's Stack Overflow posting. One of the commenters here, wubotao, pointed to this passage on the Chinese wikipedia entry for the Chinese Time Zone (中國時區)
民國17年（1928年），國民政府統一中國，原中央觀象台的業務由南京政府中央研究院的天文研究所和氣象研究所分別接收。天文研究所編寫的曆書基本上沿襲中央觀象台的做法，仍將全國劃分為5個標準時區，只是在有關交氣、合朔、太陽出沒時刻等處，不再使用北平的地方平時，而改以南京所在的標準時區的區時即東經120°標準時替代。
Emphasis by me on the key line, which may have something to do with it. This is the only entry for 1928 (which as @Henry points out, is right in the midst of the Northern Expedition unification effort).
Basically this says there is a 1928 shift in control from the former central astronomical institution (中央觀象台) to have its duties divided between the new Nanjing government's astronomical research institute (天文研究所) and its meteorology research institute (氣象研究所). While latter two based their almanac on the work of the previous one, they no longer use Beiping (now again called Beijing) standard time for time zone calculations but use Nanjing standard time
It says at 120° but that is not Nanjing but more like halfway between Shanghai (around 121°) and Nanjing (118-119°). It is, however, further east than the old capital Beijing (around 116°ish)
Not entirely clear to me exactly how this worked since I thought there would have to be some standardization with other non-Chinese timezones, but it is all I was able to find with a quick search on the Chinese side of this discussion.
I looked through the December 31, 1927 and January 1, 1928 issue of the Shanghai based Shenbao (申報) newspaper but didn't find any articles warning its readers of the coming time change.
In 1928 the Beiyang government was overthrown by the KMT. The KMT moved the capital to Nanking along with central observatory duties. Shanghai remained in the Central Plain time zone whose clock provider was still Zikawei (or Sicawei), the only change at the time I can find is that the Beijing Mean Solar Time was replaced by the Central Plain time for astronomy calendar purpose, but that was a 14-minute difference.
The almost 6 min change looks more like Zikawei’s switch from Shanghai Mean Solar Time to GMT+8, which Chinese sources described as happened in the previous century. Here’s a 1908 North-China Herald ad that mentions the Shanghai local time as GMT+8 (page 811 on https://www.google.com/books/edition/The_North_China_Herald_and_Supreme_Court/NmVMAAAAYAAJ?hl=en&gbpv=0):
SHANGHAI SEMAPHORE SERVICE. LOCAL TIME. - Shanghai mean time is reckoned from the 120th meridian, 8h. 0m 0 sec. East of Greenwich.
The Zikawei Time was also described as GMT+ 8:05:43.3 in 1906 US Navy records (https://books.google.com/books?id=QUrnAAAAMAAJ&pg=RA3-PA16#v=onepage&q&f=false). it is unclear where this 5:52 number came from.
