From a Japanese wiki site https://wiki.suikawiki.org/n/南洋群島の標準時 there are some time change history about Pacific Islands that are not included in the tz database. Mainly, the history of time used in those islands during, before and after Japanese administration, and also DST used in Guam during 1960s and 1970s.
For "Southern Islands" (modern region of Mariana + Palau + Federation of Micronesia + Marshall Islands): * A 1906 Japanese magazine shown the Caroline Islands and Mariana Islands who was occupied by Germany at the time as GMT+10, together with the like of German New Guinea. However there is a marking saying it have not been implemented (yet). No further information after that were found. * Japan invaded those islands in 1914, and records shows that they were instructed to use JST at the time * 1915 January telecommunication record on the Jaluit Atoll shows they use the meridian of 170E as standard time (GMT+11:20), which is similar to the longitude of the atoll. * 1915 February record say the 170E standard time is to be used until February 9 noon, and after February 9 noon they are to use JST. * However these are time used within the Japanese Military at the time and probably does not reflect the time used by local resident at the time (that is if they keep their own time back then) * In January 1919 the occupying force issued a command that split the area into three different timezone with meridian of 135E, 150E, 165E (JST+0, +1, +2), and the command was to become effective from February 1 of the same year. Despite the target of the command is still only for the occupying force itself, further publication have described the time as the standard time for the occupied area and thus it can probably be seen as such. ** Area that use meridian of 135E: Palau and Yap civil administration area (Southern Islands Western Standard Time) ** Area that use meridian of 150E: Truk(Chuuk) and Saipan civil administration area (Southern Islands Central Standard Time) ** Area that use meridian of 165E: Ponape(Pohnpei) and Jaluit civil administration area. (Southern Islands Eastern Standard Time) ** In the next few years Japanese occupation of those islands have been formalized via League of Nation Mandate (South Pacific Mandate) and formal governance structure have been established, these district [become subprefectures] and timezone classification have been inherited as standard time of the area. ** Saipan subprefecture include Mariana islands (exclude Guam which was occupied by America at the time), Palau and Yap subprefecture rule the Western Caroline Islands with 137E longitude as border, Truk and Ponape subprefecture rule the Eastern Caroline Islands with 154E as border, Ponape subprefecture also rule part of Marshall Islands to the west of 164E starting from (1918?) and Jalnuit subprefecture rule the rest of the Marshall Islands. * And then in year 1937, an announcement was made to change the time in the area into 2 timezones: ** Area that use meridian of 135E: area administered by Palau, Yap and Saipan subprefecture (Southern Islands Western Standard Time) ** Area that use meridian of 150E: area administered by Truk(Chuuk), Ponape(Pohnpei) and Jaluit subprefecture (Southern Islands Eastern Standard Time) * Another announcement issued in 1941 say that on April 1 that year, standard time of the Southern Islands would be changed to use the meridian of 135E (GMT+9), and thus abolishing timezone different within the area. * Then Pacific theater of WWII started and Japan slowly lose control on the island. The webpage I linked above contain no information during this period of time. Guam Island was briefly annexed by Japan during this period of time [year 1941-1944], however there are no detailed information about what time it use during that period. It would probably be reasonable to assume Guam use GMT+9 during that period of time like the surrounding area. * After the end of WWII, in 1946 February, a document written by the (former?) Japanese military personnel describe there are 3 hours time different between Caroline islands time/Wake island time and the Chungking time, which would mean the time being used there at the time was GMT+10. * After that, the area become Trust Territories of the Pacific Islands under American administration from year 1947. The site listed some American/International books/maps/publications about time used in those area during this period of time but they doesn't seems to be reliable information so it would be the best if someone know where can more reliable information can be found. * The site also pointed out that currently tzdata say Pacific/Kwajalein switched from GMT+11 to GMT-12 in 1969 October without explanation, however an 1993 article from NYT say it synchorized its day with US mainland about 40 years ago and thus the switch should occur at around 1950s instead. * Also, the NYT article linked in that website say the jump from GMT-12 to GMT+12 in year 1993 skipped Saturday that day, however the comment in tz database from that year say it skipped Friday. That seems to be an inconsistency that should be investigated, and the comment should probably be annotated accordingly if the comment was not accurate ANd then additional note for Bonin Islands: While not part of the Southern Islands, in the period of time during 1952-1968, Bonin Islands were placed under American administration. It would probably be worth investigating whether they're following the Japanese time or the Mariana time or some other time standard at the time. And then for Guam: * Currently tz database say Guam stopped using local mean time at 1901 but a 1902 earthquake report on Guam seems to be using LMT in the report when compares to the time the earthquake recorded on the Philippines (the site note that it take times for earthquake wave to transmit and thus the time different between records in different place would not be identical to the time different) ** The site say Shanks used 1901 end date for LMT time for a number of places around the world, probably using them as placeholder. Maybe those 1901 end date should be nuked from tz database? * A Japanese magazine in 1906 say Guam's time is "1.5 hours earlier than Japan" and "use 145 degree E meridian" which are contradictory to each other as 145E meridian mean GMT+9:40 and that is not 1.5 hours earlier than Japan. * And then there are some different records about Guam used GMT+9:30 at the time but each of those documents seems to give different transition time for when they give up :30 time different and move to GMT+10. But it most likely occurred before 1927 as a map published by US Navy that year shows Guam as using GMT+10, which appears to be credible as Guam was under US Military administration at the time. * As mentioned above, Guam was invaded by Japan during year 1941-1944. It's unclear about did they shifted the clock to match the surrounding islands during this period of time. * Then in year 1959 there's a daylight saving time act passed for Guam. There is a table on the site that include list an link to time and order for implementation and cancellation of DST at Guam but the implementation time seems to be all over the place according to the list. The last time the DST was implemented in Guam according to the table seems to be 1977 although I am not sure about the completeness of the table. If so then maybe a new zone for CNI would need to be created as it's after 1970 and the DST should be applicable to Guam only?