The current implementation of mcron does not provide for an at command (a command-line program that allows the user to specify that a job runs exactly once at a certain time). This can, however, be achieved.
Suppose the program
my-program needs to be run at midnight
tonight. A Guile script like the following would work (but a printed
schedule, obtained with the
--schedule option, will show
(job '(next-day) (lambda () (system "my-program") (kill (getppid) SIGINT)))