Fix Introduce a timezoneOffsetService for proposal and fix DST problem

[taiebot]

Hello all,

Thank you all for this amazing project. I did another pull request which works as well but i think it might have been not that easy to review.

Trying to fix #8040
Be aware that this is my 1st time contributing to a project like Nextcloud and i am not a developer. This work is entirely done with an AI with vibe coding over weekends. Test it carefully to see if it does not break anything else. I tried to be as close as the original code.
I introduce a timezoneOffsetService which allows to reuse it in the two parts of the code.

I managed to test this with https://github.com/szaimen/nextcloud-easy-test and what i could see and looks like it is fixed. 🤯

See original problem in #8040

With this PR

On main

Ai comparison old vs new

The old code derives the timezone offset by converting dates to locale‑formatted strings and back, which is fragile and can mis-handle daylight‑saving transitions because the string round‑trip doesn’t reliably preserve DST rules. It also depends on the system’s locale parsing behavior, which varies across environments and can introduce subtle bugs. The new code avoids these pitfalls by using Intl.DateTimeFormat with formatToParts, which is explicitly designed to respect each timezone’s DST rules. Instead of relying on string parsing, it reconstructs the exact local time components and computes the UTC equivalent using Date.UTC, producing a precise offset even during DST boundaries. The new version caches formatters for performance, while the old version recreates them on every call. Error handling also improves: the old code returns 0 on failure, masking errors, whereas the new code returns null, making issues detectable. Overall, the new implementation is more robust, DST‑safe, locale‑independent, and efficient.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[GVodyanov]

Hello! We really appreciate your PR for such an annoying issue, good job!

I see that you are having some struggles with ESLint, if you like I could fix that for you seeing as from what I can see vibe coding wasn't able to do that.

Just wanted to mention that we actually have a timezone library which might be useful here: https://github.com/nextcloud-libraries/timezones

[taiebot]

Hi @GVodyanov please feel free to fix those Linting errors they are driving mad 😠 . I am doing all this directly with the github web interface and it is not giving me any feedback until this is tested by the CI job.
I tried last night to set my self a proper working environment with Vscodium but even there i am having difficulty at setting up this environment properly.

About the timezones library comment i do not grasp the relevance. What should i do ? Do i need to do anything ?

I tried specifically to keep things as simple as i could.

My main goal was :

• to not bring any libraries : (AI kept suggesting tz.moment or others)
• Keep the code as much as possible close to the current logic (easier for review) even if some other suggestions could have been better fixes 8040 proposal DST issues #8080 works but felt too different from original logic.
• Use a function because both parts were similar
• Cross checked the functions and code logic with multiple AIs.
• Tested with https://github.com/szaimen/nextcloud-easy-test => previous versions were not tested. Note you need to refresh the browser Ctrl + Shift + R at first install to see the changes

Found another bug while testing this as well will report it and see if i can also do something about it.

Thanks again ✌️

[taiebot]

Note this bug is not triggered for current dates => Next events will get weird around 25 oct 2026 - Daylight Saving Time Ends

[github-actions]

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[SebastianKrupinski]

Hi @taiebot

Thank you for the PR, I will review it when I get a few spare minutes

[taiebot]

👍 feel free to take it from here. Hopefully it will not be a waste of your time. Just let me know if those AI tested contributions are worth it or if the code quality is subpar and you end up with more corrections than actual contribution I will keep my job of reporting bugs... ❤️