empty date in Pivot table

I want to anlayse my data from a report in a pivot table. The data contains a date field and i want to group it in the pivottabel. But when the data in this datefield contains nothing I can't group it.
How can I solve this problem?

The formula in my situation is:
=NL("Eerste";"Inkoopontvangst";"Boekingsdatum";"Projectnr.";E10;"Link=";"Inkoopontvangstregel";"Documentnr.";"=Nr.";"Projectnr.";E10;"Element";"@@"&G10;"Aantal";"<>0";"Link=";"Artikel";"Nr.";"=Nr.";"Productgroepcode";"Profiel")