Display 'Date' cell as blank instead of default year 1900

Thanks.. But doing this now - ive missed telling the whole story..
Col A - Date Completed
Col B - Date Due (in 84 days of date in Col A)
Col C - Actual Date Completed

If there is no date in A, then i want B to be blank.
If there is a date in A, then i want B to display future date i.e. =A+84
If there is a date in C then i want B to display OK

You can tell im simply a novice! Thankyou in advance.....