Why is subtracting these two epoch-milli Times (in year 1927) giving a strange result?

This is a classic Stack Overflow question regarding a peculiar Java Date/Time bug from 1927 in specific timezones, demonstrating that `ld4-ld3` is 353 seconds instead of 1. It highlights issues with historical timezone data, specifically the change from Shanghai local mean time to standard time in 1927. The problem is a specific historical anomaly rather than a general date/time library flaw.

How you'd build it

1. 1Identify the root cause of the Java `SimpleDateFormat` and `ZoneInfo` behavior for the specific date range in 1927 and the 'Asia/Shanghai' timezone.
2. 2Develop a small Java utility or library that can accurately calculate time differences across this specific historical timezone boundary, correcting for the bug.
3. 3Create a web-based micro-utility (e.g., using Next.js/React) where users can input two dates/times and a timezone, and it visually explains if the 1927 bug (or similar historical anomalies) would apply and what the 'correct' difference should be.
4. 4Implement the backend in Java (or a polyglot approach) to demonstrate the fix, perhaps using `java.time` (JSR-310) to show modern date/time handling avoids this legacy issue.
5. 5Write a detailed blog post or technical explanation on the problem, its historical context, and the solution, leveraging the web utility as a demo.

Risks & moats

• The problem is highly niche and specific to old Java versions and historical timezone data, limiting its broader appeal.
• Modern Java (`java.time`) does not exhibit this bug, making a 'fix' less relevant for current development.
• The explanation might become overly complex, requiring deep understanding of historical timekeeping and timezone changes.
• Monetization potential is extremely low as it addresses a solved problem for most contemporary systems.

Original context

If I run the following program, which parses two date strings referencing times 1 second apart and compares them: public static void main(String[] args) throws ParseException { SimpleDateFormat sf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); String str3 = "1927-12-31 23:54:07"; String str4 = "1927-12-31 23:54:08"; Date sDt3 = sf.parse(str3); Date sDt4 = sf.parse(str4); long ld3 = sDt3.getTime() /1000; long ld4 = sDt4.getTime() /1000; System.out.println(ld4-ld3); } The output is: 353 Why is ld4-ld3 , not 1 (as I would expect from the one-second difference in the times), but 353 ? If I change the dates to times 1 second later: String str3 = "1927-12-31 23:54:08"; String str4 = "1927-12-31 23:54:09"; Then ld4-ld3 will be 1 . Java version: java version "1.6.0_22" Java(TM) SE Runtime Environment (build 1.6.0_22-b04) Dynamic Code Evolution Client VM (build 0.2-b02-internal, 19.0-b04-internal, mixed mode) Timezone(`TimeZone.getDefault()`): sun.util.calendar.ZoneInfo[id="Asia/Shanghai", offset=28800000,dstSavings=0, useDaylight=false, transitions=19, lastRule=null] Locale(Locale.getDefault()): zh_CN Tags: java, date, t

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