homem shutdown (8) diz:
The first argument may be a time string (which is usually "now").
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.
Tente:
# shutdown +5
# systemctl status systemd-shutdownd.service
Você deve ver algo assim:
● systemd-shutdownd.service - Delayed Shutdown Service
Loaded: loaded (/lib/systemd/system/systemd-shutdownd.service; static; vendor preset: enabled)
Active: active (running) since Tue 2015-09-15 09:13:11 UTC; 12s ago
Docs: man:systemd-shutdownd.service(8)
Main PID: 965 (systemd-shutdow)
Status: "Shutting down at Tue 2015-09-15 09:18:11 UTC (poweroff)..."
CGroup: /system.slice/systemd-shutdownd.service
└─965 /lib/systemd/systemd-shutdownd
Status
é Shutting down at Tue 2015-09-15 09:18:11 UTC (poweroff)...