Due-date notifications for Deck cards are triggered at the wrong time

[Selduro]

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Describe the bug

Due-date notifications for Deck cards are triggered at the wrong time:

• In summer (Europe/Berlin, CEST, UTC+2): the notification fires exactly 2 hours too early.
• In winter (Europe/Berlin, CET, UTC+1): the notification fires exactly 1 hour too early.
• The offset changes precisely at the DST transition, confirming this is a UTC-offset-related issue.

Crucially, everything else that deals with the same due date works correctly:

• The due date/time is displayed correctly on the card in the user's local time.
• The "time remaining until due" value is calculated and shown correctly.

This particularly affects managed Nextcloud instances that intentionally keep the server in UTC because they host users across many timezones — e.g. Hetzner Storage Share (the environment in which I observe the bug).The hosting provider explicitly confirmed that they keep the server timezone at UTC by design and consider this a Deck bug, not a server-configuration issue.

To Reproduce

Steps:

1. Open the Deck app.
2. Create a new card on any board.
3. Set a due date/time roughly 10 minutes in the future in your local timezone.
4. Wait for the notification.

Expected behavior

The due-date notification is delivered at the configured local due time — the same time that is shown on the card itself.

Actual behavior

The notification is delivered earlier than the configured due time, offset by exactly the user's current UTC offset:

• 2 hours early during CEST (UTC+2)
• 1 hour early during CET (UTC+1)

Screenshots

Not applicable — this is a timing bug; the visual representation of the due date on the card is correct.

Client details:

• OS: Windows 11 / Android 16
• Browser: Edge + Chrome + Firefox

Server details

Operating system: Managed hosting (Hetzner Storage Share) — no shell access available to the customer.
Nextcloud version: Nextcloud Hub 25 Autumn(32.0.6)
Deck-App version: 1.16.4

Possibly related: #4764