The German naval command decided that because Roon and the other armored cruisers of the III Scouting Group were slow and lacked thick enough armor, they were unsuitable for service in the North Sea where they would risk contact with the powerful British Grand Fleet. Therefore, on 15 April 1915, Roon and the rest of III Scouting Group were transferred to the Baltic, where they would face the significantly weaker Russian Baltic Fleet. The unit was dissolved and Roon and the other vessels were assigned to the Reconnaissance Forces of the Baltic, under the command of KAdm Albert Hopman. At the same time, FK Hans Gygas replaced Karpf, who in turn became the deputy commander of the unit and made Roon his flagship. On 30 April, the ship went into drydock in Kiel for an overhaul, returning to service for the attack on Libau on 7 May. On 11 May, the British submarine E9 spotted Roon and several other ships en route to Libau, which had been recently captured by the German army. E9 fired five torpedoes at the German flotilla; two passed closely astern of Roon while the other three missed their targets as well. Roon thereafter took part in a series of sorties into the central Baltic as far north as Gotska Sandön on 13–16 May, 23–26 May, 2–6 June, 11–13 June, and 20–22 June.Karpf transferred to the light cruiser Lübeck while Hopman relocated to Roon while his flagship, the armored cruiser Prinz Adalbert, was under repairs for a torpedo hit to cover a minelaying operation with Albatross on 30 June that lasted through 2 July and resulted in the Battle of Åland Islands. The light cruiser Augsburg and three destroyers were escorting the minelaying cruiser SMS Albatross when they were attacked by four Russian cruisers—the armored cruisers Bayan, Admiral Makarov, and light cruisers Bogatyr and Oleg. Augsburg escaped while the destroyers covered the retreat of the Albatross, which was severely damaged and forced to seek refuge in neutral Swedish waters. Roon joined Lübeck to relieve the beleaguered German destroyers. Upon arriving at the scene, Roon engaged Bayan, and Lübeck opened fire on Oleg. Shortly thereafter, the Russian cruiser Rurik, along with a destroyer, arrived to reinforce the Russian flotilla. In the following artillery duel, Roon was hit several times, and the German ships were forced to retreat.Later in July, as the German Army began to push further north from Libau, the naval command reinforced the naval forces in the Baltic to support the advance. The pre-dreadnought battleships of the IV Battle Squadron were transferred to the eastern Baltic and its commander, Vizeadmiral (Vice Admiral) Ehrhard Schmidt, was placed in command of the naval forces in the area. In August, the German fleet attempted to clear the Gulf of Riga of Russian naval forces to aid the German Army advancing on the city. Elements of the High Seas Fleet were sent to strengthen the forces attempting to break into the Gulf. The Germans made several attempts to force their way into the gulf during the Battle of the Gulf of Riga until reports of British submarines in the area prompted the Germans to call off the operation on 20 August. During these attacks, Roon remained outside the gulf and on 10 August, Roon and Prinz Heinrich shelled Russian positions at Zerel on the Sworbe Peninsula. There were several Russian destroyers anchored off Zerel; the German cruisers caught them by surprise and damaged one of them.

Question: When did Germans attack the Soviets at Zerel?
Answer: 10 August