0 0 30 3 ? 2011  /command

echo /usr/bin/the_command options | at now + 1 day

0 0 2 12 * /home/adm/bin/the_command options

#! /bin/bash

COMMAND=/home/adm/bin/the_command
DONEYET="${COMMAND}.alreadyrun"

export PATH=/usr/bin:$PATH

if [[ -f $DONEYET ]]; then
  exit 1
fi
touch "$DONEYET"

# Put the command you want to run exactly once here:
echo 'You will only get this once!' | mail -s 'Greetings!' me@example.com

echo /usr/bin/the_command options | at now + 1 day

0 0 2 12 * crontab -r ; /home/adm/bin/the_command options

* * * 5 * /path/to/comand_to_execute; /usr/bin/crontab -l | /usr/bin/grep -iv command_to_execute | /usr/bin/crontab -