You first need to read the data into a matrix (you can use readtable function). 'Readtable' function will give you a table from your csv file. You can extract the GHI vector from your table as illustrated here. Suppose you have read the GHI data into a vector A. Assuming that GHI value at 12.30 is same as that at 12.00(you can change the assumptions also). You can refer to the following code
counter=13;
result=[];
for i=counter:length(A)
if(i==13)
result=[result A(i,1)];
end
if(i==(counter+24))
result=[result A(i,1)];
counter=i;
end
end
The 'result' will be your desired vector.
