Unable to plot sine wave in scope properly for large simulation time

Hi I want to plot a sine wave of 2.4 Ghz in simulink I have used frequency param=15.085e-9 and sample time=1e-10 and I have put a simulaion time =1e-6. I am unable to observe sine wave from simulation time 0, I am observing it from 0.95e-6 to 1e-6. If I reduce simulation time to 1e-7 I am able to view it properly from time t=0. Why is this so? Please help me.