Derivative of the incoming sensor data for a duration of 200ms

I need to find the derivative of accelerometer sensor data coming via the serial port in real time for a duration of 200ms.

I get 69 samples per second but the processing can delay and reduce the samples. Iam not able to figure out how do I calculate the derivative with the time. I could calculate the moving average but not the derivative. I cannot simply assume the time and go for code. I have used 11 accelerometer sensors.

Here is the code:

s = serial('COM5','BaudRate',115200); 
fopen(s);

%display data
k=1;
for j=0:inf
for i=1:34
        data(i)=str2double(fscanf(s));
        display(data);  
    pause(0.01)
end
% detect a sync bit 
 B=data(1,1);
 if B==1000
 D(k,:)=data; k=k+1;
 else
     continue;
 end  
%  for three point moving average
   if k >=4
     V=D(k-3:k-1,:);
     mv(k-3,:)=sum(V)./3;
     display(mv);
   else
     continue;  
     end
  end