Judgment of the Court (Grand Chamber) of 9 March 2021.#RJ v Stadt Offenbach am Main.#Request for a preliminary ruling from the Verwaltungsgericht Darmstadt.#Reference for a preliminary ruling – Protection of the safety and health of workers – Organisation of working time – Directive 2003/88/EC – Article 2 – Concept of ‘working time’ – Period of stand-by time according to a stand-by system – Professional firefighters – Directive 89/391/EEC – Articles 5 and 6 – Psychosocial risks – Obligation to prevent.#Case C-580/19.