Error while using ifthenelse function in SAP BODS.

Hi,

I have a source field with the data type 'datetime' that I'm
mapping it to a target field with the data type as 'varchar'.

I am using ifthenelse function to change the value to 1999.01.01 wherever
the source field value is 0101.01.01.But I'm getting the error "Invalid mapping expression for column".

Could anyone let me know where am i making mistake in the below mapping :

ifthenelse( source_table."RDAT" = 0101.01.01,'1999.01.01',source_table."RDAT")

Regards,
Ankit