As @TomMcKenzie says in a comment to another answer, date -r 123456789 is arguably a more common (i.e. more widely implemented) simple solution for times given as seconds since the Unix Epoch, but unfortunately there's no universal guaranteed portable solution.
The -d option on many types of systems means something entirely different than GNU Date's --date extension. Sadly GNU Date doesn't interpret -r the same as these other implementations. So unfortunately you have to know which version of date you're using, and many older Unix date commands don't support either option.
Even worse, POSIX date recognizes neither -d nor -r and provides no standard way in any command at all (that I know of) to format a Unix time from the command line (since POSIX Awk also lacks strftime()). (You can't use touch -t and ls because the former does not accept a time given as seconds since the Unix Epoch.)
Note though The One True Awk available direct from Brian Kernighan does now have the strftime() function built-in as well as a systime() function to return the current time in seconds since the Unix Epoch), so perhaps the Awk solution is the most portable.