There can be several ways, but the easiest might be to use a MATLAB function block: https://www.mathworks.com/help/simulink/ug/creating-an-example-model-that-uses-a-matlab-function-block.html. You can input the clock block: https://www.mathworks.com/help/simulink/slref/clock.html to the MATLAB function block and write the following code inside it
function y = fcn(t)
if t < 10
y = sin(t);
else
y = 0;
end
end
