"You know you can schedule recurring jobs with the
cron command. What if you want to run a certain
command just once, or a limited number of times, but still at times
when it is inconvenient to type in the command interactively? You
could always add the command to the crontab and then
remove it later. But there is also a tool that is made for this
job: the at command..."
"at reads commands to be executed from a file or
from standard input. You can specify the time in a number of ways,
including natural-language specifications such as
noon, midnight, or, interestingly,
teatime (which, much to the dismay of British users,
maps to 4 p.m.)..."