MADateDescription and nil objects

[Dominique Dutoit]

I have a couple of MADateDescription to manage the effective date and  
the expiration date of a structure and their values could be either a  
valid date or nil.

The problem is that once a date object is assigned to these variables  
through the fields generated by MADateDescription, it is not possible  
to assign a nil value just by deleting the content of the fields.  
MADateDescription always tries to parse the field as a date.

1. Left the field empty, effectiveDate is nil.
2. Input a date in the field, effectiveDate is a valid date.
3. Delete the content of the field, MADateDescription complains about  
an invalid date format.

Is there a way to avoid that parsing?