Wireshark for Precise Timing

Hi,

I was hoping someone could help me better understand Wireshark timing. I am trying to measure the latency of packets time tagged using a GPS clock. The device time tags the packet and then sends it to a destination. Using the top of the second packet, I am measuring the latency of the device (and simple network) by observing the time difference between when Wireshark reads the packet and the top of the second.

I have synchronized a Windows PC using a Symmetricom bus level timing card, and I believe Wireshark uses this to get its time prior to a capture. My question is how accurate is this query of the time? I believe I read somewhere that Wireshark uses the WinPcap driver to get time, and that driver has an accuracy of up to 10ms...? And during a capture, how accurate does the time remain for say 100ms? Is the inaccuracy in time constant or does it vary over a given capture?

Thank you.