I know the subtotal function will not work with horizontal ranges. My
question is what can I use to substitute it?

I have a horizontal output data table, each column represents a year
of data, and the number of years shown can change based on the users
needs (it's a cashflow statement).

What function or formula can I use in a cell at the beginning of a
given row that will add only the data visible in that given row -
disregarding the data in the hidden columns?

Thanks for the help?