[Bug]: Birthday event reminder improperly accounts for daylight savings in timezone

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

My timezone is CE(S)T and I have the problem (is not a huge deal, mostly just funny) that in the summer I get the birthday reminders at 09:00 and in the winter at 10:00.
The Android calendar app I use (Etar) handles it correctly and always shows the reminder at 09:00, so for one half per year the reminders are in sync, for the other half the reminder from Nextcloud is one hour too late.

Steps to reproduce

1. Create a contact with a birthday
2. Wait for summer :P
3. Reminder is sent at 09:00
4. Wait for winter :P
5. Reminder is sent at 10:00

Expected behavior

The reminders should always be sent at the same time, regardless of daylight savings.

Nextcloud Server version

30

Operating system

Other

PHP engine version

PHP 8.3

Web server

Nginx

Database engine version

PostgreSQL

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

List of activated Apps

Nextcloud Signing status

Nextcloud Logs

Additional info

No response

No source file, test, or entry point is named in the report. First reproduce the birthday reminder across summer and winter timezone changes, then trace the server-side scheduling path and verify that reminders are sent at the same local time in both seasons.