<Tr> <Th> Owner (s) </Th> <Td> General Motors </Td> </Tr> <Tr> <Th> Defunct </Th> <Td> April 23, 2009 (2009 - 04 - 23) </Td> </Tr> <P> Janesville Assembly Plant is a former automobile factory owned by General Motors located in Janesville, Wisconsin . Opened in 1919, it was the oldest operating GM plant when it was largely idled in December 2008, and ceased all remaining production on April 23, 2009 . </P> <P> The plant covers 4,800,000 square feet (446,000 m3). It employed around 7,000 workers at its peak in 1970, but was down to about 1,200 when it ceased production of GM vehicles in December 2008 . </P>

When did the gm plant in janesville close