missing periods

Greetings, and first of all happy and prosperous 2022.

I turn to you again to find ideas for the following scenario.

I need to build the table that is on the right of the image, starting from the table on the left, basically I need to "fill in" the spaces of missing periods between one assignment and another for each company, notice in the table on the right that it is It is necessary to identify the assignment change since the code in this case is different.
I appreciate in advance any guidance you can give me