Hello,
The timestamp that you're looking at is actually the software timestamp returned by the NI-Scope driver VI, so you're right that it will differ from when the start trigger on the digitizer hardware is received and when that first sample is returned in software, especially if you have a very long record length.
There are two properties that can help you determine a relative timestamp between records, but is actually a time value of a constantly running counter onboard the digitizer. The "Absolute Initial X" and "Relative Initial X" values that get returned as part of the "wfm Info" on the Fetch VI, will show you the timestamp of the first sample in each record of a particular acquisition and the time from the trigger to the first sample of the record, respectively. Take a look in the niScope Fetch (poly) detailed help and you can see this explanation.
These values are good for comparison between records, but not a good absolute timestamp, because this counter is constantly running and is only reset on rollover or when the board is reset. So, you can see that the values returned may look a little funny, but keep in mind they are values derived from the counter counting a certain rate. So, what you can do is take the first values returned for AbsoluteInitialX and RelativeInitialX and add/subtract them (depending on your trigger type) and make that value your "0" time, and then for each consecutive record, just extrapolate from there.
Hopefully this helps out.
Chris W
