How can i get the real time counter as Variable ?

I want to change a Variable "Vref" in Simulink with the real time and i just want the real time as a variable to write the program in matlab.

Example : for t<1 Vref = 12, ... , for t>5 Vref = 0

[Walter Roberson]

Is "t" the simulated time step, or is "t" the elapsed real-world clock time since you started running the program?