Mac OS X 10.4 has as changed the method by which the periodic system tasks periodic (daily, weekly, and monthly) are called. You can no longer change their execution time by modifying the system crontab. If you wish to change the times these tasks run, you will have to make changes to the following files. All live in /System/Library/LaunchDaemons, and each is named:
sudo pico System/Library/LaunchDaemons/com.apple.periodic-daily.plist
The relevant parts to change are:
<key>Hour</key>
<integer>3</integer>
<key>Minute</key>
<integer>15</integer>
Change these values to whatever you'd like.
Mac OS X Hints
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