Formula Help: Determine days total from arrival to current date then stop adding days

I am looking for help for a formula that calculates the number of days between an entered date and the current date and then once another cell has been filled, it will lock the days at that number and stop accumulating more days.
As in How many days has unit been in shop calculated from units arrival date to current date, but once there is a shipped date, how many days in shop stops calculating more days.

Attached is an example of my sheet that may help you understand it better since I do not think I am doing a very good job at wording it.

In E2 and copied down:

Formula:

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BSB

Try

=IF(D2="",TODAY(),D2)-B2