Detailed Instructions: In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.
Q: Passage: Childe's university position meant that he was obliged to undertake archaeological excavations, something he loathed and believed that he did poorly. Students agreed, but recognised his "genius for interpreting evidence". Unlike many contemporaries, he was scrupulous with writing up and publishing his findings, producing almost annual reports for the Proceedings of the Society of Antiquaries of Scotland and, unusually, ensuring that he acknowledged the help of every digger.His best known excavation was undertaken from 1928 to 1930 at Skara Brae in the Orkney Islands. Having uncovered a well-preserved Neolithic village, in 1931 he published the excavation results in a book titled Skara Brae. He made an error of interpretation, erroneously attributing the site to the Iron Age. During the excavation, Childe got on particularly well with the locals; for them, he was "every inch the professor" because of his eccentric appearance and habits. In 1932, Childe, collaborating with the anthropologist C. Daryll Forde, excavated two Iron Age hillforts at Earn's Hugh on the Berwickshire coast, while in June 1935 he excavated a promontory fort at Larriban near to Knocksoghey in Northern Ireland. Together with Wallace Thorneycroft, another Fellow of the Society of Antiquaries of Scotland, Childe excavated two vitrified Iron Age forts in Scotland, at Finavon, Angus (1933–34) and at Rahoy, Argyllshire (1936–37). In 1938, he and Walter Grant oversaw excavations at the Neolithic settlement of Rinyo; their investigation ceased during the Second World War, but resumed in 1946.
A:
When did Childe start the excavation that inspired his book Skara Brae?