Basically what you can do, assuming that you know the basics of Mendix:
- In domain model, create an EmployeeOverview (or something) entity with at least a datetime attribute. You can use this attribute to set the end of the period to change the details
- In domain model, associate your Employee entity with the EmployeeOverview entity (one EmployeeOverview can have 0 or more Employees and an Employee is associated to one EmployeeOverview).
- On page, show the edit button only if the current datetime <= the datetime value
- In the microflow to save employee changes, you need also check if the current datetime <= the datetime value from EmployeeOverview, because visibility conditions are not secure.