fix: Date format in sales/purchases APIs.

fix: Algorithm FIFO cost calculate method.

server/src/models/InventoryTransaction.js

@@ -12,7 +12,7 @@ export default class InventoryTransaction extends TenantModel {
   /**
    * Model timestamps.
    */
-  static get timestamps() {
+  get timestamps() {
     return ['createdAt', 'updatedAt'];
   }