O tcpdump tem suas próprias opções de timestump para.
-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.
Mais informações você pode encontrar em tcpdump manpage .