Read this: Pfizer, once the city's second largest employer, operated a large pharmaceutical research facility on the northeast side of Ann Arbor. On 22 January 2007, Pfizer announced it would close operations in Ann Arbor by the end of 2008. The facility was previously operated by Warner-Lambert and, before that, Parke-Davis. In December 2008, the University of Michigan Board of Regents approved the purchase of the facilities, and the university anticipates hiring 2,000 researchers and staff during the next 10 years. The city is the home of other research and engineering centers, including those of Lotus Engineering, General Dynamics and the National Oceanic and Atmospheric Administration (NOAA). Other research centers sited in the city are the United States Environmental Protection Agency's National Vehicle and Fuel Emissions Laboratory and the Toyota Technical Center. The city is also home to National Sanitation Foundation International (NSF International), the nonprofit non-governmental organization that develops generally accepted standards for a variety of public health related industries and subject areas.

What month in 2007 did the University of Michigan Board of Regents approve the purchase of the facilities?
What is the answer? (If it cannot be answered, return "unanswerable")
unanswerable