According to example VI the timing information is suppose to be passed to the VI in the data array. The second index, if I understand it correctly, works out to be time (he gets a delta between two points from the first index (index 0, call it dX) and then a delta between two point from the second index (index 1, call it dT), he then divides these two (dX/dT), looks an awful lot like a derivative to me).
If you are getting data from an AI Read call, that would indicate you have set up some sort of continuous sampling with AI Config and AI Start. If so, you told it what rate to sample at, and thus you already know the time between samples. Just divide the dX (from above) by the sample rate.
