Allow deadline changes in other intervals then months

[Janfred]

Especially after replying to questions from authorities it may be desirable to expand the deadline in more fine-grained intervals than months.

Example: https://fragdenstaat.de/a/234616

The authority asked for clarification and a decision regarding the fee. After securing that the fee would be covered by an association, I replied. Since the authority was waiting for me, I want to expand the deadline by the time it took me to respond.

This is especially useful afterwards when reminding the authority of the missed deadline with the exact number of days.

[stefanw]

I'm open to this. Currently, you can only set the deadline ahead in a unit of time specified by the law (1 month for German Federal FOIA, but e.g. days for German press law).

A date input to specify an arbitrary future date.
This would make it impossible to set accurate deadlines (1 month extension calculated like here).

Choosing the unit of time when extending the deadline, with units of "month", "working days"
This could work, but makes it difficult to specify concrete date deadlines if they are given.

Please also note, that authorities regularly stall by sending canned letters back, asking questions that should not need to be asked. We assume the starting date of a request is the day after it was received, not when any unnecessary back-and-forth has been resolved.