My date format is dd.mm.yyyy

In this case OpenOffice destroys dates and Excel does not open the file at all, shows error.
To fix this I changed IsNumber() to

LPARAMETERS tcContents
LOCAL llIsNumber
tcContents = ALLTRIM(tcContents)

DO CASE 
   CASE OCCURS('.', tcContents)>1
		llIsNumber = .F.

	CASE LEN(tcContents) = 0
		llIsNumber = .F.
	CASE LEN(CHRTRAN(tcContents,'0123456789.,$','')) = 0
		llIsNumber = .T.
	OTHERWISE
		llIsNumber = (LEFT(tcContents,1) = '-' AND LEN(CHRTRAN(SUBSTR(tcContents,2),'0123456789.,$','')) = 0)
ENDCASE
RETURN llIsNumber