The following options are available:
-h The system is halted at the specified time.
-r The system is rebooted at the specified time.
-k Kick everybody off. The -k option does not actually halt the
system, but leaves the system multi-user with logins disabled
(for all but super-user).
-o If -h or -r is specified, shutdown will execute halt(8) or
reboot(8) instead of sending signal to init(8).
-n If the -o is specified, prevent the file system cache from being
flushed by passing -n option to halt(8) or reboot(8). This
option should probably not be used.
-u The system is halted up until the point of removing system power,
but waits before removing power for 5 minutes so that an external
UPS (uninterruptible power supply) can forcibly remove power.
This simulates a dirty shutdown to permit a later automatic power
on. OS X uses this mode automatically with supported UPSs in
emergency shutdowns.
time Time is the time at which shutdown will bring the system down and
may be the word now (indicating an immediate shutdown) or specify
a future time in one of two formats: +number, or yymmddhhmm,
where the year, month, and day may be defaulted to the current
system values. The first form brings the system down in number
minutes and the second at the absolute time specified.
etc...