Moving from Cognos impromptu to Cognos Express

I am having an issue in transferring a date expression into Cognos Express

if ([customer/sales/invoice/parts].[MBD050].[POSCHREQD]<[customer/sales/invoice/parts].[MBD020].[GRNDATE]) then (([customer/sales/invoice/parts].[MBD050].[POSCHREQD] - [customer/sales/invoice/parts].[MBD020].[GRNDATE]) * -1) else (000 00:00:00.0000)

this keeps failing but works in Impromptu can anyone tell my why!!!