Why effective start date can not be determined in Manage individual compensation task?

Summary:

Content (required):

Why it is not possible to determine effective start when adding Individual Compensation as Line Manager:

In the same time it is possible when we perform this action under Administer Compensation as HR Specialist?

Can the effective date be editable in both cases?

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):