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.
In the View menu click Time Display Format and choose one of the Time of Day options.
-t
Don't print a timestamp on each dump line.
-tt
Print the timestamp, as seconds since January 1, 1970, 00:00:00, UTC, and fractions of a second since that time, on each dump line.
-ttt
Print a delta (micro-second resolution) between current and previous line on each dump line.
-tttt
Print a timestamp, as hours, minutes, seconds, and fractions of a second since midnight, preceded by the date, on each dump line.
-ttttt
Print a delta (micro-second resolution) between current and first line on each dump line.
(from comment)
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.
In the
View
menu clickTime Display Format
and choose one of theTime of Day
options.tcpdump has its own timestump options for.
More info you can find at tcpdump manpage.