Prevent Termination Date to be greater than Sysdate + *amount* days

Hi experts,

we need to prevent employees from uploading resignation (Resign from employment action) with a termination date greater than sysdate + 30 days.

As I understood it's not possible to display only specific dates in the calendar pop-up (right?), we need to develop an error Validation rule with VBS.

I VBS we do find the termination date field, but we are not getting to comparing it with the sysdate and have the system calculates sysdate + 30 days.

Would you please help? Is it feasible in the first place?

Thanks in advance

Domenico