On the date(1) command and seconds since 1970
Russell Coker wrote something about converting between timestamps since the epoch and human-readable dates.
I haven't fully parsed his post because I'm still half asleep, but I feel the need to point out the @ operator that I discovered last year in newer versions of GNU coreutils.
apollock@icarus:~$ date +%s 1212672691 apollock@icarus:~$ date -d @$(date +%s) Thu Jun 5 06:31:50 PDT 2008
It seems a bit shorter than what Russell was using, and presumably does the same thing.