MATLAB gradient derivative troubleshooting
You can calculate the derivative using the diff function and then dividing by the time interval between the "samples" in your vector like this:

dA = diff(A) / 286


The diff() function calculates the difference between an element and the previous one in the vector. With this you'll be calculating the derivative for each element in the vector as the increment in A divided by the increment in x. Check out the help for the diff() for more info.