How can I do a delayed poweroff in linux sort of like the shutdown command? But I want it to actually poweroff and not just halt.

If it's not possible, how can I send a broadcast message the same way the shutdown command does? Since I could just have a sleep loop do it, and use the shut down command after. It's for a backup server that only turns on when needed and shuts off when the job is done, but I want the delay to enable me to do something if I decide to and the alerts to remind me it's shutting down so I can cancel the shutdown if needed.

Have a look at "man shutdown" - that will tell you all you need to know.