in case the above does not work for you. for me this did not for some reasons;
the UNIX numbers i am working on are from the Mozilla place.sqlite dates.
to make it work : i splitted the UNIX cells into two cells : one of the first 10 numbers (the date) and the other 4 numbers left (the seconds i believe)
Then i used this formula, =(A1/86400)+25569 where A1 contains the cell with the first 10 number; and it worked