I want date to = the day the status = completed. How to change a field ""Completed Date"" to the date of when a multiple choice field, containing ""Completed"" is selected?

If the Status field is not logged, you could do this:

If(
[Status]="Completed",ToDate([Date Modified])
)

The danger is if the record is modified again on a later date, the later date will show.

If the Status field is a logged field, you can use Left and Right statements to isolate the date in the text of the logged field.