Many standard utilities (rdist, make, cron, etc.) allow you to specify a command to run at a certain time. Usually, this command is simply passed to the Bourne shell, which means that you can execute whole scripts, should you choose to do so.
Lastly, Unix runs Bourne shell scripts when it boots. If you want to modify the boot-time behavior of a system, you need to learn to write and modify Bourne shell scripts.