Judgment of the Court (Tenth Chamber) of 23 January 2019.#K.M. Zyla v Staatssecretaris van Financiën.#Request for a preliminary ruling from the Hoge Raad der Nederlanden.#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.#Case C-272/17.