The first line written gives the time that timestamp was started, in milliseconds since the epoch. Note that logging events can cause timings to change, and that the timestamps are only as accurate as the times reported by /dev/time.
% {echo hello; sleep 1; echo goodbye} | timestamp eg 0000000000 eg start 1080156139468 0000000007 eg hello 0000000988 eg goodbye
TIMESTAMP(1 ) | Rev: Tue Mar 31 02:42:38 GMT 2015 |