From the moment it's plugged in, the DAQ is acquiring a frame every ~50 or ~33 ms (depending on the acquisition rate you've set), and this is happening even if you are not saving these frames on the PC (i.e. "recording"). So, when you click "record", or trigger recording externally, that moment will likely fall somewhere in the middle of the ~50 to ~33ms period. So, the software will register the last frame acquired before you started recording as frame 0 at time which is a negative number because it represents the time before the beginning of the recording session. The first "recorded" frame will be frame 1 at the positive amount of time since the beginning of the recording session.
My answer is based on what I understood from an answer to a similar question some time ago, if someone wants to correct me please go ahead.
So, you can ignore frame 0 with the negative timestamp for analysis purposes.