I think one of the first answers here was the answer but I didn't use the DTOS function correctly. so I can do this:
dbf1.Active:=false;
dbf1.Exclusive:=true;
dbf1.Open;
dbf1.AddIndex('custdate','field1+'+'DTOS(datefield)',[ixCaseInsensitive]);
dbf1.Close;
dbf1.Exclusive:=false;
dbf1.Active:=true;
This appears to work now, thanks for everyone's help here