It sounds like you want to plot duration values. Let's say that t is your vector of datetime values. You can convert those to duration values by subtracting:
t = datetime('now')+minutes(0:0.5:10);
d = t-t(1); % Convert to duration since start of experiment.
d.Format = 's'; % Specify that you want to see seconds in the display.
Once you've converted, you can either plot the values directly:
plot(d, 1:21)
Or you can explicitly convert to seconds before plotting:
plot(seconds(d), 1:21)
xlabel('seconds')
