Shutdown delay in Devuan Linux is made the same way as shutdown delay in other Linux distros. The only thing you have to do is to run "shutdown" comman with proper parameters. This is very easy. For example this command will delay system shutdown for 20 minutes:
$ shutdown +20
In case you want to delay shutdown for 2 hours, you run:
$ shutdown +120
This is simple thing: just set a number of minutes.
You already know, that to shut down system immediately, you can run a following command in linux terminal:
$ shutdown now
System will shutdown immediately. Surely, it you want to shutdown and then reboot it, you will run:
$ shutdown -r now
But sometimes you have to plan your shutdown (corporate rules, system maintenance, parental control etc). In this case you have just to pospone system shutdown using proper parameters while running "shutdown" command.
Sometimes you need to lock system and don't let users to log in. One of the solutions is to schedule system shutdown and postpone it with a lock from new logins. It can happen in case you want to upgrade the system, but it's impossible to kick logged users from the system and you want just to let them finish their work and then do you work.
Just imagine you need to postpone shutdown for 20 minutes and let users know that system maintenance is coming. Let's start postponed shutdown with an announcement, what is going to happen:
$ shutdown +20 "System upgrade is coming. Thank you for your patience"