Referring to SuiteAnswers ID #30950, the solution provided does not work.

Production Date is a Custom Transaction Line Field of type Date.

Shelf Life is an Integer field.

--- BEGIN CODE ---

        var prodDate = currentRecord.getCurrentSublistValue({

          sublistId: sublistName,

          fieldId: 'custcol_prod_date'

        });

        log.audit({

          title: 'Production Date',

          details: prodDate

        });

        var shelfLife = currentRecord.getCurrentSublistValue({

          sublistId: sublistName,

          fieldId: 'custcol_item_shelf_life'

        });

        log.audit({

          title: 'Shelf Life',

          details: shelfLife

        });

        var nProdDate = format.parse({

          value: prodDate,

          type: format.Type.DATETIMETZ

        });

        log.audit({

          title: 'Formatted Production Date',

          details: nProdDate

        });

        var expDate = nProdDate.setMonth(nProdDate.getMonth() + shelfLife);

        log.audit({

          title: 'Expiration Date',

          details: expDate

        });

--- END CODE ---