Velocity graph(Shift in sampling time)

Hi,

I am trying to do position to velocity differentiation of discrete points along X-axis and carry out the frequency response of the signal by FFT. Please check the attachment for velocity vs time graph. From the velocity over time graph, sampling starts at X = 1.907s. What does it signifies? Why the sampling doesn't start at 0?.. Sampling rate or time is = 0.0017 sec and number of samples are 25000. Any help would be grateful. Thank you