% Define some pretend data, the size of your original matrix
M = rand(62,50,341);
% Intervals
interval = [8,30,31,31,30,31,30,31,31,28,31,29];
% Get the sizes of M and intervals
[r,c,~] = size(M);
ni = length(interval);
% Calculate endpoints
endpoints = cumsum(interval);
% Calculate starting points
startpoints = [1 endpoints(1:end-1)+1];
% Preallocate an array for the summed intervals
output = zeros(r,c,ni); % Could also get these values from the sizes of the inputs
% Fill the output with the summed intervals
for ii = 1:ni
output(:,:,ii) = sum(M(:,:,startpoints(ii):endpoints(ii)),3);
end
