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Every 30 data rows, I would average them out and save it in 1 row.This way, 30 days shrinks down to 1 data (a month). Or another way, you can simply delete 29 data rows and keep only one from every 30 data rows.
Given thirty days in every month try this
%% daily_val = rand( 30*12, 1 ); %% daily_val = reshape( daily_val, 30,  ); %% monthly_val = sum( daily_val, 1 );
here I assume that the first value in daily_val is the first value of a month. This is intended as a start.