Scheduling a process to execute later:

# at   now   +2    minute
at>reboot
(press ctrl + d to save & exit)

# at 07:10 PM February 28
at>reboot
(ctrl + d)

 

Few Example (of at command):
# at 8:00pm December 7

# at 7 am Thursday
# at midnight + 23 minutes
# at now + 5 minutes

 

atq
(to list job scheduled)

 

at -l
(to list job scheduled)

 

at -d jobnumber      
(to delete the job)

 

Crontab file format:
# vim  /etc/crontab (see man 5crontab)
02   9   28   2   *   root      /sbin/reboot
(reboot system at 9:02 on 28 Feb)

0    7   2   4     0    root       /bin/mkdir   /wali/abc
(create directory at 7:00 am 2nd April, Sun)

1    14   2   4   1    root       /bin/rm  -rf   /wali/xyz 
(removes directory at 2:01 pm 2nd April, Mon)

(save & exit)