When reporting an error always report the whole error message including the line at which it occurs.
A guess is that one of your date30 is not member of date_sep2 and therefore one of the m element is 0. As you're using m(i, 1) as an index into out this causes problem.
If that is the case, the proper to fix this depends on what you want to do when a date30 is not found.
As an aside, you're taking a risk using length in your code. length is going to return the number of columns if there are less rows than columns. This would be a lot more robust since it will work regardless of the numbers of rows in date30:
date30_all = NaN(size(date30, 1), size(data, 2)+6); %and no need for extra brackets around date
I would recommend never using length. Use size with an explicit dimension for matrices, and numel for vectors.
As an other aside, since m is a vector, I would write m(i) instead of m(i, 1). That way it does not matter if m is a row or column vector.
