Without know much about your import, does the date field always get it's info by import? If so, you could just put your calc into the auto enter calc. on the date field. Keep in mind now your date field may not be a true date field and you may need to use the GetAsDate function if you need this field for other calculations
Second option would be to import to a temp table, use the auto enter calc to correct the data before putting it into your date field.
I would use one field for the imported text and a true date field with an auto-enter calculation that takes the imported text and produces a date using either getasDate or Date() functions. If you enable auto-enter options during import, you can simply import the data and your date field will show the correct, converted date.