How to Define Item Rule that assigns or validates value in not-editing row

Summary:

How to Define Item Rule that assigns or validates value in not-editing row

Content (please ensure you mask any confidential information):

We setup following EFFs in Attribute Group of "Unit Price Master".

Date of Start (Date), Date of End (Date), Unit Price (Number)

I would like to implement the following requirement.

When entering new row of "Unit Price Master" EFFs, Item Rule assigns or validates "Date of End" of old row.

When Users create new Item, they usually enter only "Date of Start" and "Unit Price".

They don't enter "Date of End".

When Users enter new row of "Unit Price Master", "Date of End" should be auto-completed or validated.