Change duration handling of date options

[dartcafe]

⚠️ This issue respects the following points: ⚠️

• I agree to follow Nextcloud's Code of Conduct.

Describe the goal you'd like to achieve

To avoid further duration misinterpretation, change the handling of durations.

Describe possible solutions

• Add a real duration compatible between luxon and PHP
• Add new duration column to db
• Migrate duration to new value (respect DST situations)
• later on: remove old duration

Additional context

No response

[github-actions]

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