Update date column and copy value from a different date column only once

Hi,
Looking for a way to update column date and copy date from a different date column only once.

Not sure if it’s automation (preferably) or formula,
but the scenario is that column date 1 is not necessarily being created with a date value, but when it does being updated with one,
I want to copy it over to the other date column for tracking and comparison purpose (original date vs most recent date).

Hope this was clear enough, thanks in advanced.