The at facility submits a command line (or a script) for execution at an arbitrary later time. It has the form:
%atoptions time < scriptfile
This submits the 
scriptfile
 for execution at a later 
time
. The redirection (
<
) isn't required on BSD and some other UNIX systems. If you don't want to write a script, you can omit it and type your commands on the terminal, terminated by  CTRL-d:
%atoptions time Command 1 Command 2... [CTRL-d]
The 
time
 is most commonly a four-digit number representing a time on a 24-hour clock. For example, 
0130
 represents 1:30 a.m. and  
1400
 represents 2 p.m. You can also use abbreviations such as  
1am
, 
130pm
, and so on.
- from O'Reilly & Associates' System Performance Tuning , Chapter 3