#8 specify how often a task should be run

[Zen Scient]

Allow a task to specify the frequency at which it
should be run. E.g. once a day, week, month, etc.
Each time the task completes successfully (that is,
without logging any messages >= ERROR), a timestamp is
wrtten (to preferences?); each time the task execution
is requested, the time of the most successful execution
is queried to determine if the task should be run or not.

Note that this probably requires task names in order to
provide a key with which the timestamp can be
associated in the persistent store.