Time stamps, their precisions and all that can be quite confusing. This section will provide you with information about what's going on while Wireshark processes time stamps.
While packets are captured, each packet is time stamped as it comes in. These time stamps will be saved to the capture file, so they also will be available for (later) analysis.
So where do these time stamps come from? While capturing, Wireshark gets the time stamps from the libpcap (WinPcap) library, which in turn gets them from the operating system kernel. If the capture data is loaded from a capture file, Wireshark obviously gets the data from that file.
The internal format that Wireshark uses to keep a packet time stamp consists of the date (in days since 1.1.1970) and the time of day (in nanoseconds since midnight). You can adjust the way Wireshark displays the time stamp data in the packet list, see the "Time Display Format" item in the Section 3.7, “The "View" menu” for details.
While reading or writing capture files, Wireshark converts the time stamp data between the capture file format and the internal format as required.
While capturing, Wireshark uses the libpcap (WinPcap) capture library which supports microsecond resolution. Unless you are working with specialized capturing hardware, this resolution should be adequate.
Every capture file format that Wireshark knows supports time stamps. The time stamp precision supported by a specific capture file format differs widely and varies from one second "0" to one nanosecond "0.123456789". Most file formats store the time stamps with a fixed precision (e.g. microseconds), while some file formats are even capable of storing the time stamp precision itself (whatever the benefit may be).
The common libpcap capture file format that is used by Wireshark (and a lot of other tools) supports a fixed microsecond resolution "0.123456" only.
Writing data into a capture file format that doesn't provide the capability to store the actual precision will lead to loss of information. Example: If you load a capture file with nanosecond resolution and store the capture data to a libpcap file (with microsecond resolution) Wireshark obviously must reduce the precision from nanosecond to microsecond.
It's often asked: "Which time stamp accuracy is provided by Wireshark?". Well, Wireshark doesn't create any time stamps itself but simply gets them from "somewhere else" and displays them. So accuracy will depend on the capture system (operating system, performance, ...) that you use. Because of this, the above question is difficult to answer in a general way.
USB connected network adapters often provide a very bad time stamp accuracy. The incoming packets have to take "a long and winding road" to travel through the USB cable until they actually reach the kernel. As the incoming packets are time stamped when they are processed by the kernel, this time stamping mechanism becomes very inaccurate.
Conclusion: don't use USB connected NIC's when you need precise time stamp accuracy! (XXX - are there any such NIC's that generate time stamps on the USB hardware?)