drupal 6: how to email users of tasks due (week b4, day before, and overdue)

Using open atrium, have tasks with assigned to (user开发者_开发技巧 reference) field, due date field, neither field required. Also using flag to note if task is completed or not.

Want to email users a reminder of tasks when that task's due date is a week away, again when it's the day before, and each day when it is overdue.

If task has a due date, if due date is X-time away or past due, if task is not complete, if task is assigned to someone, then email reminder to their registered email address.

Tried doing a rule to run on cron, but don't see this type of options. Looked at scheduler, auto expire, node expire, they don't work the way I need.

Ideas?

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You should take a look at the signup module. It does what you ask for. If you can't / wont use it, you can take a look at how it handles the sending of email reminders.