Since Japan has exactly one timezone and most services are intended for use within Japan, many Japanese programmers are unaware of timezone-related logic. I've frequently encountered Japanese sites store dates as OffsetDateTime instances (rather than UTC Instants), and have frequently seen subtle bugs, like a web app's backend returning UTC+9 timestamps, and then subsets of the frontend format the date according to the user's local time. I witnessed this last week on a fairly big concert ticket website. The lottery application datetime was simply formatting the OffsetDateTime returned by backend. Then, for some reason, the lottery result announcement datetime was being passed through a different formatting function. Two adjacent timestamps were in different timezones and there were no visual indications of this. I learned a lot about the site's API when investigating. I later reported the bug and was told the service is not intended for use outside of Japan, so they won't fix.

I've also encountered games where device local time results in bugs when certain features are gated to durations within Japanese Standard Time. For one particular game, I contacted support and the response was similarly "this service is intended for use within Japan; please ensure your device time is set to Japanese Standard Time." The support representative actually went further and told me, "if you are using this service outside of Japan, then we cannot make any guarantees on [the app's] behavior or resolve issues."

To most programmers in the US or Europe, storing all dates as UTC and localizing datetimes in the frontend may seem like "common sense", but this is essentially 異文化 to many Japanese programmers, for lack of a better way to phrase it.