SYNOPSIS
   /sbin/shutdown [-akrhPHfFnc] [-t sec] time [warning message]

[...]

   time   When to shutdown.

shutdown -h 21:45

sudo apt-get install at

$ echo "ls > a.txt" | at now + 1 min
warning: commands will be executed using /bin/sh
job 3 at Thu Apr  4 20:16:00 2013

$ echo "ls > a.txt" > cmd.txt
$ at now + 1 min < cmd.txt
warning: commands will be executed using /bin/sh
job 3 at Thu Apr  4 20:16:00 2013

$ at now + 1 min
warning: commands will be executed using /bin/sh
at> ls

$ at -t 201403142134.12 < script.sh

*     *     *     *     *  command to be executed
-     -     -     -     -
|     |     |     |     |
|     |     |     |     +----- day of week (0 - 6) (Sunday=0)
|     |     |     +------- month (1 - 12)
|     |     +--------- day of month (1 - 31)
|     +----------- hour (0 - 23)
+------------- min (0 - 59)

04 14 * * * ls

34 14 15 5  /path/to/command

echo "shutdown +5" | at 10:05am 2019-01-19

shutdown -r +1440

$ sudo shutdown -h 12:00

-h, -P, --poweroff

-r, --reboot

-c