How to refer to a previous timestep in a matrix?

I have several timesteps and a large matrix (lets call it old) I would like to replace the old matrix with an updated version containing the "old" from the previous timestep and another matrix called "new". I would like to do this for every row and column.

My problem is how to refer to the row and column in the previous timestep. How can I do that? The bolded part...

for t=2:481

      for row=1:231;
          for col=1:204;
  old(row,col)= *old(t-1)* -new(row,col);
  end
  end
  end

Answer 1

[Walter Roberson]

old(row,col) = old(row,col) - new(row,col);

In the particular case you show, you can eliminate the loop and use

old(1:231, 1:204) = old(1:231, 1:204) - new(1:231, 1:204);

If those are the complete array limits rather than just part of the array, this can be recoded as

old = old - new;

[Johanna]

Thanks!