[Bug]: Unused DateTimeImmutable::modify() return value in CalDavBackend.php generates PHP warning

⚠️ 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

This warning has been captured directly from the nextcloud logs:

"The return value of method DateTimeImmutable::modify() should either be used or intentionally ignored by casting it as (void), as DateTimeImmutable::modify() does not modify the object itself at /var/www/nextcloud/apps/dav/lib/CalDAV/CalDavBackend.php#3437"

Because PHP Manual for DateTimeImmutable::modify returns a brand-new object instead of changing the original one, calling it without using the result is usually a coding mistake.

Maybe it could be fixed by changing

$date->modify('+1 day');
to
$date = $date->modify('+1 day'); so you actually save the changed date.

Steps to reproduce

1. Every time the function is called (when webdav is used).

Expected behavior

I expect no warning is generated whenever the library is used.

Nextcloud Server version

34

Operating system

Other

PHP engine version

PHP 8.5

Web server

Apache (supported)

Database engine version

PostgreSQL

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

Updated from a MINOR version (ex. 32.0.1 to 32.0.2)

Are you using the Nextcloud Server Encryption module?

Encryption is Disabled

What user-backends are you using?

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

Configuration report

{
    "system": {
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
            "***REMOVED SENSITIVE VALUE***",
            "***REMOVED SENSITIVE VALUE***"
        ],
        "allowed_admin_ranges": [
            "X.X.X.X\/24"
        ],
        "trusted_proxies": "***REMOVED SENSITIVE VALUE***",
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "overwrite.cli.url": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "pgsql",
        "version": "34.0.2.1",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbport": "",
        "dbtableprefix": "oc_",
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "ldapIgnoreNamingRules": false,
        "ldapUserCleanupInterval": 51,
        "ldapProviderFactory": "\\OCA\\User_LDAP\\LDAPProviderFactory",
        "mail_from_address": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpmode": "smtp",
        "mail_domain": "***REMOVED SENSITIVE VALUE***",
        "default_phone_region": "ES",
        "maintenance": false,
        "theme": "",
        "loglevel": 2,
        "onlyoffice": {
            "verify_peer_off": true
        },
        "mail_smtphost": "***REMOVED SENSITIVE VALUE***",
        "updater.release.channel": "stable",
        "trashbin_retention_obligation": "10, 30",
        "mysql.utf8mb4": true,
        "memcache.local": "\\OC\\Memcache\\Redis",
        "memcache.locking": "\\OC\\Memcache\\Redis",
        "memcache.distributed": "\\OC\\Memcache\\Redis",
        "redis": {
            "host": "***REMOVED SENSITIVE VALUE***",
            "dbindex": 0,
            "port": 6379,
            "timeout": 0
        },
        "app_install_overwrite": {
            "0": "passman",
            "1": "bruteforcesettings",
            "2": "files_markdown",
            "4": "libresign",
            "5": "twofactor_email",
            "6": "socialsharing_facebook"
        },
        "memories.vod.path": "\/var\/www\/nextcloud\/apps\/memories\/bin-ext\/go-vod-amd64",
        "memories.vod.ffmpeg": "\/usr\/bin\/ffmpeg",
        "memories.vod.ffprobe": "\/usr\/bin\/ffprobe",
        "memories.vod.disable": false,
        "memories.vod.vaapi": true,
        "mail_sendmailmode": "smtp",
        "memories.db.triggers.fcu": true,
        "maintenance_window_start": 1,
        "preview_max_x": 1024,
        "preview_max_y": 1024,
        "jpeg_quality": 80,
        "enabledPreviewProviders": [
            "OC\\Preview\\Imaginary"
        ],
        "preview_imaginary_url": "***REMOVED SENSITIVE VALUE***",
        "files.chunked_upload.max_size": 10485760,
        "mail_smtpport": "25",
        "memories.gis_type": 2,
        "memories.exiftool_no_local": true,
        "serverid": "10"
    }
}

List of activated Apps

Enabled:
  - activity: 7.0.0
  - admin_audit: 1.24.0
  - announcementcenter: 7.4.1
  - app_api: 34.0.0
  - approval: 3.3.1
  - appstore: 1.0.0
  - bruteforcesettings: 7.0.0
  - calendar: 6.5.1
  - camerarawpreviews: 1.1.2
  - circles: 34.0.0
  - cloud_federation_api: 1.18.0
  - comments: 1.24.0
  - contacts: 8.7.4
  - contactsinteraction: 1.15.0
  - dav: 1.39.0
  - deck: 1.18.3
  - drawio: 4.3.5
  - epubviewer: 1.9.7
  - federatedfilesharing: 1.24.0
  - federation: 1.24.0
  - files: 2.6.0
  - files_accesscontrol: 5.0.0
  - files_automatedtagging: 5.0.0
  - files_downloadlimit: 5.2.0
  - files_external: 1.26.0
  - files_lock: 34.0.1
  - files_pdfviewer: 7.0.0-dev.0
  - files_reminders: 1.7.0
  - files_retention: 5.0.0
  - files_sharing: 1.26.0
  - files_trashbin: 1.24.0
  - files_versions: 1.27.0
  - firstrunwizard: 7.0.0-dev.0
  - forms: 5.3.5
  - groupfolders: 22.0.5
  - logreader: 7.0.0
  - lookup_server_connector: 1.22.0
  - memories: 8.1.0
  - nextcloud_announcements: 6.0.0
  - notes: 6.0.1
  - notifications: 7.0.0-dev.1
  - notify_push: 1.3.5
  - oauth2: 1.22.0
  - office: 1.0.0
  - onlyoffice: 10.1.2
  - password_policy: 6.0.0-dev.0
  - photos: 7.0.0
  - polls: 9.1.4
  - previewgenerator: 5.14.0
  - privacy: 6.0.0-dev.1
  - profile: 1.3.0
  - provisioning_api: 1.24.0
  - quota_warning: 1.24.0
  - recommendations: 7.0.0
  - related_resources: 5.0.0-dev.0
  - serverinfo: 6.0.0
  - settings: 1.17.0
  - sharebymail: 1.24.0
  - socialsharing_email: 4.1.0
  - socialsharing_facebook: 4.1.0
  - spreed: 24.0.3
  - support: 6.0.0
  - survey_client: 6.0.0-dev.0
  - suspicious_login: 12.0.0-dev.0
  - systemtags: 1.24.0
  - tasks: 0.18.1
  - terms_of_service: 4.7.1
  - text: 8.0.0
  - theming: 2.9.0
  - twofactor_backupcodes: 1.23.0
  - twofactor_email: 3.3.0
  - twofactor_totp: 16.0.0
  - twofactor_webauthn: 2.7.0
  - updatenotification: 1.24.0
  - user_ldap: 1.25.0
  - user_status: 1.14.0
  - viewer: 7.0.0-dev.0
  - weather_status: 1.14.0
  - webhook_listeners: 1.6.0
  - workflowengine: 2.16.0
Disabled:
  - dashboard: 7.14.0 (installed 6.0.0)
  - encryption: 2.22.0
  - files_markdown: 2.4.1 (installed 2.4.1)
  - files_rightclick: 0.15.1 (installed 1.6.0)
  - impersonate: 5.0.0 (installed 5.0.0)
  - integration_mattermost: 3.1.0 (installed 3.1.0)
  - integration_overleaf: 2.0.1 (installed 2.0.1)
  - jitsi: 0.19.0 (installed 0.19.0)
  - maps: 1.7.1 (installed 1.7.1)
  - tables: 2.2.0 (installed 2.2.0)
  - talk_matterbridge: 1.34.1026000 (installed 1.34.1026000)
  - testing: 1.23.0
  - twofactor_nextcloud_notification: 8.0.0
  - user_external: 4.0.0 (installed 4.0.0)

Nextcloud Signing status

No errors have been found

Nextcloud Logs

{"reqId":"EVrtz8Jy0KEsnW4vzKyP","level":2,"time":"2026-07-24T18:30:06+00:00","remoteAddr":"","user":"--","app":"PHP","method":"","url":"--","scriptName":"/var/www/nextcloud/cron.php","message":"The return value of method DateTimeImmutable::modify() should either be used or intentionally ignored by casting it as (void), as DateTimeImmutable::modify() does not modify the object itself at /var/www/nextcloud/apps/dav/lib/CalDAV/CalDavBackend.php#3437","userAgent":"--","version":"34.0.2.1","occ_command":["/var/www/nextcloud/cron.php"],"data":{"app":"PHP"},"id":"6a64638788c39"}

Additional info

No response

The warning points to apps/dav/lib/CalDAV/CalDavBackend.php at line 3437. Read the surrounding date-handling code first and determine whether the DateTimeImmutable::modify() result should be retained or intentionally ignored. Done means the PHP warning is eliminated without changing the intended CalDAV behavior; run the relevant DAV or CalDAV checks available in the repository.