XML error: writing date into nil field in InfoPath (xsi:nil) with Data Event

I'm using a Data Event to keep a running log of who approved the last task and when.

However when I write the date into the form, I get an XML error because the field is no longer nil, e.g.:

<my:authorisedDate xsi:nil="true">8/12/2009 5:15:48 PM</my:authorisedDate>

How do I clear the nil on this field?