sum up cells based on cell indices (row and column NUMBERS)

I have a spreadsheet for calculating attendance % based on the last 24 weeks. So I have a cell with today's date, and I use this to find the index of the cell with the closest previous date, simple enough so far....

What I'd like to do is sum up the total of the previous 24 columns. So currently I'm on column 34 based on today's date, how do I sum up columns 21:34, is there a way of doing this without converting to cell letters? All of the cell number/letter conversions I found only go up to Z