Script to suspent my nas?

Hi guys,
I want to create a script that will suspent my nas. Right now I already have a script that will automatically wake up my NAS into "scheduled job tab" (NOT power mangement->scheduled job)
/usr/sbin/rtcwake -m disk -s 64800
So what command should I use to choose the time of the shutdown?
I found those commands:
echo 'pmi action suspend' | at 3am
echo pm-suspend | at 3am (do I need to put "sudo" before at?)
is it right? Whyt this use the 3am format and not the seconds? Also, can I write 3:27pm?