Actually, Tiger does a thoroughly lousy job of that periodic tasks, unless they've significantly fixed launchd lately. Anacrond does a good job, but they unfortunately tried to replace crond with launchd, including a halfbaked implementation of what anacron does.
The periodic tasks are only picked up if the computer is asleep, not if it's turned off. So if you turn your computer on in the morning and off in the evening every day for a month, the periodic tasks will never run.
The periodic tasks are also only run once per boot, so if your uptime is a month, the "daily", "weekly" and "monthly" jobs will each be run only once.