Calculation of Integral square error from Simulink

I have a control system model in simulink. Using "to workspace" block, I have taken the output y and time t from the clock to Matlab workspace. I wish to find error in a similar way as given below. The problem is that t is not having equal intervals. deltat is not constant from the clock data. If deltat is constant, then I can find integral square error. What should I do?

err = 1-y;
deltat = 0.1;
ise = trapz(deltat,err.^2);