Add Visible Columns

[MikeDBMan]

Across the top in Row 15 I have 2012.Jan, 2012.Feb, etc repeated 17 times for each of 17 entities. I wish to add up all the 2012.Jan columns THAT ARE VISIBLE. What would be an easy way to do this. Assume the columns begin in Column D and extend through the next 200+ columns, GZ. Thanks!

 

[MikeDBMan]

Note that I am talking about visible columns not filtered columns of data. Thanks.