You can add tasks without a due date in two different ways:

Remind me to read Jane Goodall's book

When Siri responds by asking, "When would you like me to remind you?" you can say:

Whenever

Or, you can add the task in another way:

Add "Read Jane Goodall's book" to my Remember The Milk list.

When you phrase it this way, Siri won't automatically add a date or ask you for one.

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