The specification for "at" is as specified in the SUS but with the following differences as listed below.
cats the jobs listed on the command line to standard output.
This option is functionally equivalent to the -r option specified in the SUS.
This option is not supported but the '-d' option is equivalent.
This option is not supported.
Prints the version number to standard error.
Shows the time the job will be executed.
The files at.allow and at.deny reside in /etc rather than /usr/lib/cron.