How to create a report of records that have missing date?

[Guido1982]

I need to create a report of assets and filter those on a custom date field. The filter criterium is that the field should be empty (it is not mandatory). We want to create a list of assets where that date is missing. But in reports, I can't filter on empty dates since an empty string is not a valid date format. I also tried a dash '-' and '00-00-0000', but without success. How can I filter reports on empty date fields?