Given the rationale, provide a reasonable question and answer. Step-by-step reasoning process: To answer the question, consider the following: On July 16, 2009, the building was officially renamed Willis Tower.
The question and answer:
Although Sears' naming rights expired in 2003, the building continued to be called the Sears Tower for several years. In March 2009, London-based insurance broker Willis Group Holdings agreed to lease a portion of the building, and obtained the building's naming rights. On July 16, 2009, the building was officially renamed Willis Tower. On August 13, 2012, United Airlines announced it would move its corporate headquarters from 77 West Wacker Drive to Willis Tower.

When did the sears tower became the willis tower?
The answer is July 16 , 2009