Case C-580/19: Judgment of the Court (Grand Chamber) of 9 March 2021 (request for a preliminary ruling from the Verwaltungsgericht Darmstadt — Germany) — RJ v Stadt Offenbach am Main (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)