Why Does Java’s Date Show an 8‑Hour Shift? Uncovering Timezone Pitfalls

Online Issue

New feature marketing activity was supposed to end at 23:59:59, but at 16:00 the page already showed "activity ended".

Investigation Process

HTTP packet capture

activityEndTime = 1654761599000, which converts to 2022-06-09 15:59:59, 8 hours earlier than expected. Not a frontend issue.

Database query

Activity end time stored in two fields:

endDate (dateTime): 2022-06-09
endTime (time): 23:59:59

Java receives both as Date, no issue observed. Possibly time type to Date conversion?

Cache inspection endDate=1654704000000, endTime=57599000 Data flow DB → Java Date → JSON serialization is correct.

Thus the only possibility is an error when concatenating endDate and endTime into activityEndTime.

Concatenation logic

activityEndTime = endDate.getTime() + endTime.getTime();

Debug shows this is problematic.

Calling Date.getTime() returns milliseconds since 1970‑01‑01 00:00:00 GMT. For UTC+8, this shifts 8 hours left.

Therefore, when adding endDate (midnight) and endTime (23:59:59) timestamps, the result is offset by an extra 8 hours.

Demo code:

Date date1 = new Date(0L); // 1970‑01‑01 00:00:00 GMT, 08:00 in UTC+8
Date date2 = new Date(3600*1000L); // 01:00 GMT, 09:00 UTC+8
Date date3 = new Date(7200L*1000); // 02:00 GMT, 10:00 UTC+8
Date date4 = new Date(date1.getTime() + date2.getTime());
Date date5 = new Date(date1.getTime() + date2.getTime() + date3.getTime());
System.out.println(date4);
System.out.println(date5);

Problem Cause

The missing 8 hours stem from the fact that the stored date and time are in the local timezone (UTC+8), but getTime() converts them to UTC timestamps, causing a left shift of 8 hours. Adding them together shifts only once to the right, resulting in an overall 8‑hour loss.

4. Date’s Timezone Determination

Java determines the current timezone from the system property user.timezone; if unset, it falls back to native methods based on java.home. Typically the timezone configuration resides in /etc/localtime or the zoneinfo database.

Source: 2021不再有雨, blog.csdn.net/w727655308/article/details/125211726