In the View menu click Time Display Format and choose one of the Time of Day options. While pcap is supported in Wireshark/TShark as well, their default format is now the pcap Next Generation Capture File Format (pcap-ng). A pcap file (from tcpdump or wireshark or AFAIK anything else using libpcap) already has absolute time it's only the Wireshark display you need to adjust. Some of the notable variants of pcap are Wireshark’s nanosecond libpcap (nseclibpcap), the modified tcpdump-libpcap (modlibpcap), Nokia’s tcpdump-libpcap (nokialibpcap), and various Linux implementations. Each captured packet starts with the timestamp in seconds, the timestamp in microseconds, the number of octets of packet saved in file, and the actual length of the packet. This information is followed by zero or more records of captured packet data.

The global header contains the magic number, GMT offset, timestamp precision, the maximum length of captured packets (in octets), and the data link type. Although this format varies somewhat from implementation to implementation, all pcap files have the general structure shown in Fig. The pcap file format is a binary format, with support for nanosecond-precision timestamps. The de facto standard network packet capture format is libpcap (pcap), which is used in packet analyzers such as tcpdump/WinDump and Wireshark.

Tcpdump wireshark pcap format