Right, so.
You have the days (12) in cell A1 (for example), and you have a drop down box in C1 (for example). When you select anything in the dropdown box add the days that are at the end to the days in A1?
If so:
=TRIM(LEFT(A1,SEARCH(" day",A1,1)))+TRIM(MID(C1,SEARCH("+ ",C1,1)+1,SEARCH(" day",C1,1)-SEARCH("+ ",C1,1)))
Where A1 is your day value (12) and C1 is your dropdown box.