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Script Timer seems to be one option. The author writes, "Yes, you can schedule a job to run only once in Script Timer. It is treated as a special case for a repeating job - you just set the repeating time interval to zero."
You can schedule scripts/commands from iCal by creating an event and setting an alarm to "Run Script". Optionally, if you want to execute some commands regularly - you should really look into using Cronjobs. Cronjobs let you specify command which will be executed at specific time(s). There are various GUI frontends for managing cronjob on Mac: Cronnix - ...
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