I realize this answer is far to late to be of use for when you asked the question, but this question is still getting many views so I am going ot go ahead and answer it. There are two primary ways to measure time in Simulink. If you are using digital control, you can use a resetable counter, https://www.mathworks.com/help/simulink/slref/counterlimited.html?searchHighlight=counter&s_tid=srchtitle_support_results_7_counter
or, you can use a continuous time integrator with a reset/enable state turned on. This lets you use it to measure the time between events and you can control it with rising/falling edges.
