Case C-272/17: Judgment of the Court (Tenth Chamber) of 23 January 2019 (request for a preliminary ruling from the Hoge Raad der Nederlanden — Netherlands) — K.M. Zyla v Staatssecretaris van Financiën (Reference for a preliminary ruling — Free movement of workers — Equal treatment — Income tax — Social security contributions — Worker who left the Member State of her employment during the course of the calendar year — Application of the pro rata temporis rule to social security credit)