EG's solution is certainly simple and elegant, and it works well in my application.
In order add columns past the last date, I added an additional blank column (after Column J) and used this as the identifyer for the ranges, thus:
Dates $B$2:$K$2
ColCount $B$3:$K$3
LastCell $K$3
I just have to remember to drag the formula in Row 3 across to the new column.
(My red box now looks for the last date, and if older than 3 months, highlights red.)
Thanks for everyone's tremendous assistance.
