ARIS PPM: Calculated attribute Default value (TIME type)

Hi,

I have one question about default value for calculated date (TIME) attributes.

This attribute can be NULL at the source system.  I want to mantain and show  its NULL (empty) value.

But, it seems to be mandatory to have a default value for the calculated attribute.

Q: Is there a way NOT to have a default value for calculated DATE attributes or I must have a convention value (i.e. 01-january-1970) ?

Thanks in advance.