CAB Cutoff Validation for Normal Changes

[Sirri]

Hi Team,
I have a requirement related to CAB Date/Time (cab_date_time) on the Change Request table.
Scope

Applicable only for Normal Changes.
CAB meetings are conducted on:

Monday
Thursday

Business Requirement
A cutoff time exists for adding changes to the upcoming CAB meeting.
Cutoff time: 16:00 (4:00 PM) on the previous CAB-working day.
Examples:
Scenario 1 – Monday CAB

CAB meeting is on Monday.
Cutoff time is Sunday 16:00.
If a user tries to select a Monday CAB Date/Time after Sunday 16:00, the system should:

Show an error message:

"Time to include a change in CAB has passed, please follow CAB Cutoff exception procedure"

Clear the value selected in the cab_date_time field.

Scenario 2 – Thursday CAB

CAB meeting is on Thursday.
Cutoff time is Wednesday 16:00.
If a user tries to select a Thursday CAB Date/Time after Wednesday 16:00, the system should:

Show an error message:

"Time to include a change in CAB has passed, please follow CAB Cutoff exception procedure"

Clear the value selected in the cab_date_time field.

Exception
Users with the role:
change_manager
should be able to select any CAB Date/Time regardless of the cutoff, with no error displayed.
Configurability Requirement
The solution should use System Properties so that CAB schedules and cutoff times can be managed without code changes.
Suggested properties:

CAB Days
Example: Monday,Thursday

CAB Cutoff Time
Example: 16:00

Questions

What would be the recommended approach:

Client Script + GlideAjax?
onChange Client Script?
Business Rule validation?
Combination of client and server validation?

How would you design the solution using System Properties for:

CAB meeting days
CAB cutoff time

Has anyone implemented a similar CAB cutoff validation before?

Please provide the entire script.

Thanks in advance.