This task involves annotating the answer type to a given question that involve some kind of complex reasoning (including numerical reasoning). Note that the questions require looking at more than one part of the passage to answer. There are 3 possible answer types (i) spans, (ii) numbers and (iii) dates. If the answer can be found in the passage, label it as "span". If the answer is a number, label as "number". Similarly, label "date" if you think the answer to the given question is a date.

Passage: The unit was split into two regiments in December 1939, with Fegelein in charge of both. By March 1941 their strength was 3,500 men. In July 1941, they were assigned to the Pripyat swamps (punitive operation), tasked with rounding up and exterminating Jews and partisans. The two regiments were amalgamated into the SS Cavalry Brigade on 31 July, twelve days after the operation started. Fegeleins final report, dated 18 September 1941, states that they killed 14,178 Jews, 1,001 partisans, and 699 Red Army soldiers, with 830 prisoners taken. The historian Henning Pieper estimates the actual number of Jews killed was closer to 23,700. The SS Cavalry Brigade took serious losses in November 1941 in the Battle of Moscow, with casualties of up to 60 per cent in some squadrons. Fegelein was appointed as commander of the 8th SS Cavalry Division Florian Geyer on 20 April 1943. This unit saw service in the Soviet Union in attacks on partisans and civilians. In addition, SS Cavalry regiments served in Croatia and Hungary.
Question: What date did the punitive operation begin?
date