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There is a new feature in 10.4 (tiger): the "Energy Saver" can now be scheduled to wake most macs from sleep. In eariler versions like 10.2 (jaguar) and 10.3 (panther), you could schedule a power-up at a recurring time (like 3am every day), but that would NOT wake a box that was powered-on but sleeping. Now, in 10.4, with the ability to schedule a power-on or wake-up, with judicious choice of settings, you can pretty much guarantee certain certain scheduled tasks always are run, such as the 3 periodic task groups (daily, weekly, and monthly).
For example, suppose I want to ensure that these 3 periodic task-groups always run. I think the following should work (I have yet to verify):
1. In Energy-Saver, "Put the computer to sleep when it is inactive for:" 3 hrs (not next highest value, "Never").
2. In Schedule, set "Start up or wake: Every Day: at 3:05am". The daily periodic tasks start at 3:15am by default.
3. I think the periodic-weekly should also be editted to run at 4:30 (as it did in Panther), also discussed at URL below.
4. Re-schedule the periodic-monthly to run at 4:45am instead of 5:30am (Tiger uses /System/Library/LaunchDaemons (instead of /etc/crontab) so you must edit com.apple.periodic-monthly.plist as shown at URL below: