How to clear a field using a Workflow Rule

I have added a custom Date field to Projects to capture the date when a project is Cancelled.  I have setup a Workflow Rule to set the date when the project becomes Cancelled, and I want to setup a corresponding Workflow Rule to clear the date field when the project changes from Cancelled to some other state.  But I can't find how to express the empty value for the field in the Action dialog (a simple empty string '' is not permitted).  The field seems to expect a DateTime value, but I want the field to be empty.

I thought of using the GetPreviousValue() function for the field, but if the value had previously been manually adjusted it would not then be cleared.