Abstract: In data processing of a computer software having ambiguous data such as a data structure with date field having an ambiguous YY two digit year representation based on 00, inputting of one or more ambiguous year representations is detected prior to performing a processing step and a corresponding unambiguous year representation with the same format as the ambiguous year representation is substituted for each detected ambiguous year representation in performing the processing step. One or more unambiguous year representations to be outputted by the processing step is detected and each unambiguous year representation to be outputted is converted to the corresponding ambiguous year representation which is outputted. The substituted unambiguous YY year representation has the same format as the YY ambiguous year representation but uses a base year other than 00 that precedes the earliest date of the ambiguous date fields.