Need some help with VBA.

I have two columns each with dates (see below). I'm looking for a VBA
function that will look at each date and provide another date based on
the following criteria: It needs to first look at Column B and
figure out 180 days from that date, then compare that value to Column
C to ensure that the date is still 180 days from Column C, if not,
then calculae that date. Basically I'm looking for a multi function
conditional formula that will get me a date in Column D that is always
180 days from Column A, B, C based on the inputs into each cell.


Column A Column B Column C Column D
7/10/09 6/30/10 (1 year from Column A) (insert value
from VBA)
9/1/10 3/31/10