This is very simple to install. Ntpclient will update your slug's clock for you. Although it doesn't appear SlugOS loses time that much, this is probably a good thing to install.
ipkg install ntpclient
That's it. It will automatically start when the nslu2 boots. It will update every 600 seconds (10 minutes) by default.
The init script is located at /etc/init.d/ntpclient
Edit that file to change the settings.