Convert epoch timestamp (seconds since 1970) on the command line

Use this to convert epoch timestamp (seconds since 1970) on the command line. For some reason the man page doesn't reveal this secret:

date -d @1241958464

If you have the epoch seconds in hex, use this on linux:

echo $((0x4f91d6f8)) | xargs -I## date -d @##

The same thing is much easier on OS X:

date -r 0x4f91d6f8