Sum calculaton needed, Help!

Hi,

Trying to carry out a relatively simple date calculation, as follows

Date is held in cells in Column A
Column D is A1+14 to give a date 14 days later, ie Target Date

Thats the simple bit, but I need to add a variable column C, for which if data is entered to a cell will influence the result in Column D and reset the target date.

This should reflect a change in target date, so for example if the original date is 08/12/2011 + 14 should equal 22/12/2011. However, if 10/12/2011 was entered in Column C the target date would need to be able to handle the difference and return 24/12/2011.

Any help gratefully received.

In column D, enter this formula:

=if(c1="",a1+14,c1+14)

Copy this down column D.