Auto-generate Name of User

I don't know if you are using Legacy forms or new style forms, but you should be able to set up a form rule that triggers when the record is saved, and the date field has changed, then take the action to change the value of the field for who updated the date to the current user.  make the field of who  change the date a user type field..

If the form rule doesn't natively offer up the current user as a choice to put in that field, then you can make a helper field called current user as a formula user type with the formula of User(). Then have the form rule change the value of the target field two the value in that current user field.