[Bug]: Event without timezone causes push notification at UTC

⚠️ 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

An event with DTSTART:20240125T110000 but no timezone info show correctly in Calendar for a user living in Berlin but causes a push notification one hour too late because the server operates in UTC.

Steps to reproduce

1. Save event with notification but without timezone

Expected behavior

Calendar timezone or global timezone will be used as fallback when the event has no timezone.

Installation method

None

Nextcloud Server version

28

Operating system

None

PHP engine version

None

Web server

None

Database engine version

None

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

No response

List of activated Apps

No response

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

No response

Start by reproducing the issue in Nextcloud Server 28 with an event using DTSTART:20240125T110000 and a notification but no timezone. Compare the Calendar display with the push-notification time for a user in Berlin while the server uses UTC. Done means the event's Calendar or global timezone is used as the fallback for notification scheduling.