Listbox or dynamic list

well i have a worksheet like this:


in the B1, C1, D1 E1, etc i have dates
in A2, A3, A4, A5, A6, etc i have error codes and in B2, B3,C2, C3,D2,D# i have the values of the amount of that error codes in that dates.

this worksheet is like a matrix, you have the values of the error codes for each date.

in another sheet i want to have a dropdown list with the values of B1,C1,D1,E1, but this list has to be dynamic because im adding each day another date and i dont want to extend the range all the time.
i find this:

=OFFSET($A$2,0,0,COUNTA($A$2:$A$200),1)

but i think this list only works when you have the values of the list in A1,A2,A3, and not in A1, B1, C1.

other thing i want to know is if i put a listbox on the other sheet, when i insert a new date can i add by code a value to a listbox in another sheet? (i think this is more portable and "updatable")

thanks for all!