The time string may either be in the format "hh:mm" for hour/minutes
specifying the time to execute the shutdown at, specified in 24h clock
format. Alternatively it may be in the syntax "+m" referring to the
specified number of minutes m from now. "now" is an alias for "+0",
i.e. for triggering an immediate shutdown. If no time argument is
specified, "+1" is implied.
(adapted from man page 'shutdown')