Reminder for 1976 in the event - iOS/macOS Calendar

Steps to reproduce

1. Create an event in non-personal calendar in native Calendar app either on iOS or macOS
2. Add a reminder
3. Check the event in web app - additional reminder set on 1st Apr 1976 00:55 (UTC) is set

Expected behavior

There should be no additional reminder visible on web app from year 1976

Actual behavior

Hello team,

when I create an event in a native Calendar app either on iOS or macOS and add a reminder in it, there is an additional reminder set on 1st April 1976 0:55 (UTC). When I create an event and add a reminder through web or another CalDAV client (i.e. built-in calendar in Vivaldi browser) there is no additional reminder set in the event, which is visible only in the web app. This behaviour affects only events with reminders in non-personal calendars.

This might be a client issue, but I face this issue only in non-personal calendars. Is it possible to check?

Similar/same issue was reported already in 2021 and it's already closed as non-reproducable.
https://github.com/nextcloud/calendar/issues/3054

Thank you in advance for your time spent on checking this!

Calendar app version

6.1.6

CalDAV-clients used

macOS/iOS Calendar native app

Browser

Vivaldi 7.8.3925.56

Client operating system

macOS 26.2

Server operating system

Ubuntu 24.04.3 LTS

Web server

Apache

Database engine version

MariaDB

PHP engine version

PHP 8.3

Nextcloud version

32.0.5

Updated from an older installed version or fresh install

Updated from an older version

List of activated apps

Enabled:
  - activity: 5.0.0-dev.0
  - app_api: 32.0.0
  - bruteforcesettings: 5.0.0-dev.0
  - calendar: 6.1.6
  - circles: 32.0.0
  - cloud_federation_api: 1.16.0
  - comments: 1.22.0
  - contacts: 8.3.1
  - contactsinteraction: 1.13.1
  - dashboard: 7.12.0
  - dav: 1.34.2
  - federatedfilesharing: 1.22.0
  - federation: 1.22.0
  - files: 2.4.0
  - files_downloadlimit: 5.0.0-dev.0
  - files_fulltextsearch: 32.0.2
  - files_pdfviewer: 5.0.0-dev.0
  - files_reminders: 1.5.0
  - files_sharing: 1.24.1
  - files_trashbin: 1.22.0
  - files_versions: 1.25.0
  - firstrunwizard: 5.0.0-dev.0
  - fulltextsearch: 32.0.0
  - fulltextsearch_elasticsearch: 32.0.2
  - logreader: 5.0.0-dev.0
  - lookup_server_connector: 1.20.0
  - nextcloud_announcements: 4.0.0-dev.0
  - notifications: 5.0.0-dev.0
  - oauth2: 1.20.0
  - password_policy: 4.0.0-dev.0
  - photos: 5.0.0-dev.1
  - previewgenerator: 5.11.0
  - privacy: 4.0.0-dev.0
  - profile: 1.1.0
  - provisioning_api: 1.22.0
  - recommendations: 5.0.0-dev.0
  - related_resources: 3.0.0-dev.0
  - serverinfo: 4.0.0-dev.0
  - settings: 1.15.1
  - sharebymail: 1.22.0
  - support: 4.0.0-dev.0
  - survey_client: 4.0.0-dev.0
  - systemtags: 1.22.0
  - text: 6.0.1
  - theming: 2.7.0
  - twofactor_backupcodes: 1.21.0
  - twofactor_webauthn: 2.4.1
  - updatenotification: 1.22.0
  - user_status: 1.12.0
  - viewer: 5.0.0-dev.0
  - weather_status: 1.12.0
  - webhook_listeners: 1.3.0
  - workflowengine: 2.14.0

Nextcloud configuration

ovcacikt@nuc-ubuntu-nextcloud:~$ sudo -u www-data php /var/www/nextcloud/occ config:list system
{
    "system": {
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
            "nuc-ubuntu-nextcloud.lan",
            "lordran84.dynu.net:8443",
            "ltmbo.duckdns.org:8443"
        ],
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "mysql",
        "version": "32.0.5.0",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbport": "",
        "dbtableprefix": "oc_",
        "mysql.utf8mb4": true,
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "overwrite.cli.url": "https:\/\/nuc-ubuntu-nextcloud.lan",
        "installed": true,
        "default_phone_region": "cz",
        "memcache.local": "\\OC\\Memcache\\APCu",
        "enable_previews": true,
        "enabledPreviewProviders": [
            "OC\\Preview\\PDF",
            "OC\\Preview\\Image",
            "OC\\Preview\\Photoshop",
            "OC\\Preview\\TIFF",
            "OC\\Preview\\SVG",
            "OC\\Preview\\Font",
            "OC\\Preview\\MP3",
            "OC\\Preview\\Movie",
            "OC\\Preview\\MKV",
            "OC\\Preview\\MP4",
            "OC\\Preview\\AVI"
        ],
        "mail_smtpmode": "smtp",
        "mail_sendmailmode": "smtp",
        "mail_smtpsecure": "ssl",
        "mail_from_address": "***REMOVED SENSITIVE VALUE***",
        "mail_domain": "***REMOVED SENSITIVE VALUE***",
        "mail_smtphost": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpport": "465",
        "mail_smtpauthtype": "LOGIN",
        "loglevel": 2,
        "mail_smtpauth": 1,
        "mail_smtpname": "***REMOVED SENSITIVE VALUE***",
        "mail_smtppassword": "***REMOVED SENSITIVE VALUE***",
        "maintenance": false,
        "theme": "",
        "share_folder": "\/Sd\u00edlen\u00e9",
        "updater.release.channel": "stable",
        "app_install_overwrite": [
            "previewgenerator",
            "fulltextsearch",
            "fulltextsearch_elasticsearch",
            "files_fulltextsearch"
        ],
        "maintenance_window_start": 1
    }
}

Web server error log

Log file

Browser log

Additional info

No response

No source files or tests are named. First reproduce the issue with a native iOS or macOS Calendar client against a non-personal calendar, then trace how the web app handles the imported reminder. Done means the event no longer displays an unintended reminder dated 1 April 1976, without breaking valid reminders.