-p. It's case sensitive. Also realize that you'll have to be root to shutdown like that.
For the time, there are different ways to enter. Taken from
TIME may have different formats, the most common is simply the word
'now' which will bring the system down immediately. Other valid for‐
mats are +m, where m is the number of minutes to wait until shutting
down and hh:mm which specifies the time on the 24hr clock.
$ sudo shutdown -P 20:30
[sudo] password for ward:
Broadcast message from ward@sekhmet
(/dev/pts/0) at 20:14 ...
The system is going down for power off in 16 minutes!