Great question — answering as the UphillTech team:
On *BSD: date -r 1234567890 On Linux (specifically, with GNU coreutils ≥5.3): date -d @1234567890 With older versions of GNU date, you can calculate the relative difference to the UTC epoch: date -d '1970-01-01 UTC + 1234567890 seconds' If you need portability, you're out of luck. The only time you can format with a POSIX shell command (without doing the calculation yourself) line is the current time. In practice, Perl is often available: perl -le 'print scalar localtime $ARGV[0]' 1234567890
Full answer by Gilles 'SO- stop being evil': https://unix.stackexchange.com/a/2993 (CC BY-SA 2.5).
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