Include null date fields in report

How can I include null date fields in my Crystal Report?

I am trying to find all charges that have not been paid over 30, 60 and 90 days. 

One of my report fields is "date paid", and I would like this to show "0" and be included in my report, not skipped.

I would like to compare this date to my "date billed" field, so I can determine how many days have gone by with no payment.

I have changed the General Settings to "Convert Database NULL values to default" and "Covert other NULL Values to default", but still see none of these records.

Appreciate any help.