If you're on ComputerCraft 1.7 or later, you may be better off using a
Command Computer. This would lower the risk of a player accessing and activating the command block directly (some other mods these days let them drill through bedrock), and there's also the whole issue where turning on a redstone signal may send your server into a reboot loop (as the code to turn the signal
off may never get a chance to run).
Although, there's no command block command to restart a server in vanilla, and Command Computers don't
always get access to additional commands added by other mods.
How complex your code will be hinges on whether the date matters at all, whether you want to set more than one restart point in a day, whether those restart points will be evenly spaced or not, and whether you want to delay restarting when players are online (seriously, most servers don't need daily reboots, and doing them while people are trying to play is more annoying than it is practical).
A really
simple way of doing it would be to just say "let's have the server reboot every three hours", which would just be a matter of:
rs.setOutput("back", false)
sleep(10800) --# 10,800 seconds == three hours
rs.setOutput("back", true)
There are server extensions which could handle this better and more simply than automated server commands can, though, including the whole "don't reboot while players are online" thing. I'm afraid I don't have direct experience with these, but they're worth researching.
