time is the time of day, so it doesn't contain information about the date, only the hours, minutes, seconds within a day. In order to plot the whole thing, you can add time to the datetime array date1 and plot glucose against the result:
%Plot of entire patient one time series
load ('glucose.mat')
plot(date1+time,glucose),
xlabel('Time'), ylabel('Glucose')
title('Glucose readings for each time of the day')
%Finding the mean of the glucose readings for the time series
M = mean(glucose)
%Finding the standard deviation of the glucose readings for the time series
S = std(glucose)
