[Lazarus] Filtering on date with TDbf

[Koenraad Lelong]

Hi,

Lazarus 0.9.30

I want to filter a TDbf. The field is a date.
When I try in Filter in Lazarus :
DATUM >= '1/1/2011'
  or
DATUM >= "1/1/2011"
  or
DATUM >= '1.1.2011'

In my application :
tblTik.Filter:='DATUM>='''+DateToStr(XferStart)+'''';

I get an error :
Index based on unknown field ">=".

What am I doing wrong ?

Regards,

Koenraad Lelong.