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
superfluous entries).
(job '(next-day)
(lambda () (system "my-program")
(kill (getppid) SIGINT)))