Given the task definition and input, reply with output. This task is about writing a correct answer for the reading comprehension task. Based on the information provided in a given passage, you should identify the shortest continuous text span from the passage that serves as an answer to the given question. Avoid answers that are incorrect or provides incomplete justification for the question.

Passage: The principal Treaties that form the European Union began with common rules for coal and steel, and then atomic energy, but more complete and formal institutions were established through the Treaty of Rome 1957 and the Maastricht Treaty 1992 (now: TFEU). Minor amendments were made during the 1960s and 1970s. Major amending treaties were signed to complete the development of a single, internal market in the Single European Act 1986, to further the development of a more social Europe in the Treaty of Amsterdam 1997, and to make minor amendments to the relative power of member states in the EU institutions in the Treaty of Nice 2001 and the Treaty of Lisbon 2007. Since its establishment, more member states have joined through a series of accession treaties, from the UK, Ireland, Denmark and Norway in 1972 (though Norway did not end up joining), Greece in 1979, Spain and Portugal 1985, Austria, Finland, Norway and Sweden in 1994 (though again Norway failed to join, because of lack of support in the referendum), the Czech Republic, Cyprus, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia and Slovenia in 2004, Romania and Bulgaria in 2007 and Croatia in 2013. Greenland signed a Treaty in 1985 giving it a special status. Question: When did Denmark join the EU?
1972