DueDate reported with wrong timezone in frontend API

I don't know if this is exactly the rather old issue #2276...
But I still have the same problem on Nextcloud 26.0.2 (docker) with TZ=Europe/Vienna.

I'm working on a feature for creating Deck Cards in QOwnNotes (https://github.com/pbek/QOwnNotes/issues/2789).

When I use the Deck API to create a Card, I send:

{"duedate":"2023-06-06T06:15:30+02:00","order":0,"title":"tz4","type":"plain"}

I get back:

{"ETag":"8ef788d31fae586d508563b5b52c5de1","archived":false,"assignedUsers":null,"attachmentCount":null,"attachments":null,"commentsCount":0,"commentsUnread":0,"createdAt":1686024933,"deletedAt":0,"description":"","descriptionPrev":null,"duedate":"2023-06-06T06:15:30+02:00","id":78,"labels":null,"lastEditor":null,"lastModified":1686024933,"notified":false,"order":0,"owner":"test","relatedBoard":null,"relatedStack":null,"stackId":16,"title":"tz4","type":"plain"}

Note that the timezones are both correct, 2023-06-06T06:15:30+02:00.

But when I look at the Deck page on my Nextcloud server, the timezone is not correct at all!

2023-06-06T06:15:30+00:00

I can't see any useful Nextcloud Log errors/warning in that period.

Start by reproducing the issue with the documented Deck API payload and compare the returned duedate with the value shown on the Deck page. Trace how the frontend handles the API date and timezone; done means a +02:00 due date is displayed correctly rather than as +00:00, with the behavior verified against this example.