One way:

=IF(B1<"",B1,D1) - A1

Where D1 contains your "specific date"

In article ,
Debbie wrote:

Hi, I have 2 date columns. One is a start date (column A), the other column
contains withdrawl dates(column B) (many cells in this column are blank). In
the third column I want to determine the number of days between column B and
column A but if column B is blank I want to use a specific date to subtract
from the start date.