What is you're actually wanting to do? Are you wanting to logoff the server, have it reboot at a scheduled time and start up and reconnect to a shared drive?
The subst command only associates a (nice short) drive letter with a (horribly long) path. It doesn't map a drive to another server.
You can use a scheduled task to map the drive at startup: "net use x: \\server\share". You have to use an appropriate user account to get access to the share. I would give the local admin account rights to the share, or a domain account used just for that purpose. If you use a domain account, make sure you give it rights to "log on as batch job" or "service" or any of those other things on the server you're working from.
You can do exactly the same thing using a batch file and an AT command, if you want to just specify a time.
If this isn't what you're intending to do, could you clarify it?