What is the problem exactly?
You have your signal u 3001x1, with a sampling time of 10ms. It means that u(1) is the value of the signal at t=0s and u(3000) is the signal at t=30s. So u(800) is the value at time = 8 sec, and u(800:1500) is the value between time t=8 sec to 15 seconds.