Populate expiration date based on pre-defined offset for patron group

Background:
In Settings, a user can define a default offset in days for the expiration date of each patron group. this is optional for each patron group.
If the patron group does have a defined offset, the expiration date field can be filled with the current date plus the given offset.
This automatically entered value can be edited or deleted by the user: it is only a help for choosing the right date, no restriction.

Scenarios:
Users App, Create/Edit Screen.

(1)
Given: No patron group is selected.
Then: No button is displayed.

(2)
Given: A patron group is already selected (In edit screen)
(2a)
Given: The patron group has no offset defined in settings.
Then: No button is displayed.
(2b)
Given: The patron group does have an offset defined in settings.
Then: The button "Recalculate" is displayed underneath the field "expiration date".
When: The user clicks the button
Then: The expiration date is changed to the current date (server time) plus the number of days defined in the settings as offset.

(3)
Given: The user selects a patron group in the dropdown.
(3a)
Given: The patron group has no offset defined in settings.
Then: No button is displayed.
(3b)
Given: The patron group does have an offset defined in settings.
Then: A modal appears:
"Users with patron group group usually expire after offset days.
Do you want to change the expiration date accordingly?"
Options: "Cancel" - "Recalculate"
When: The user clicks the button "Cancel", Then: The field Expiration date remains unchanged.
When: The user clicks the button "Recalculate". Then: The expiration date is changed to the current date (server time) plus the number of days defined in the settings as offset.
After closing the modal, see (2b): The button still displays, because the user could manually change the filled in date and might want to recalculate again.

In addition:

Given: An expiration date that is already entered an lies in the past, the status is "inactive", the information text "User has expired" is displayed in red underneath the field status.
When: The expiration date field is changed (manually or via button "Recalculate") to a value >= the current date
Then: An information text apperas instead of or underneath the "User has expired" info, reading: "User will reactivate after saving"