You don't include any code (which would tell us how you are doing this and what version of LabVIEW you are using), but I'm going to guess you are using an Express VI whose output, "Signal", is a (Dreaded) Dynamic Data Wire (I added the third "D" descriptor). You can turn this into a Waveform using the Signal Manipulation palette, and if you know how to use Waveforms, you can find the value that is nearest to your chosen time. If the Signal Generator is sending out "packets" that are shorter than your chosen time, you'll need concatenate them or otherwise handle the pieces.
Why don't you give it a try, see if it makes sense to you, and if you have more questions, come back to this post and (please!) attach your VI. [If you only attach a picture of the VI, I'll probably complain ...].
Bob Schor
