Data acquisition toolbox TriggerTime accuracy

I have some trouble with the TriggerTime I get returned when doing background sampling of an analog signal with the ni usb-6009 in matlab. It looks like it is off by quite alot, and it also seems to vary based on what value I set for the NotifyWhenDataAvailableExceeds parameter. Is this timestamp set in sw? If so, what is the accuracy of this timestamp? I really need to know the absolute time when the first sample was taken, but this seems difficult!?