Please answer the following question: Information:  - Expedition 28 was the 28th long-duration expedition to the International Space Station, and began on 23 May 2011 with the departure of the members of Expedition 27. The first three members of Expedition 28 arrived on the ISS aboard the Soyuz TMA-21 spacecraft on 4 April 2011, and were joined on 9 June 2011 by the three other crew members, who arrived aboard Soyuz TMA-02M. The expedition saw a number of significant events, including the final Space Shuttle mission, STS-135, which took place in July 2011. Expedition 28 was superseded by Expedition 29 on 16 September 2011.  - Expedition 29 was the 29th long-duration expedition to the International Space Station (ISS). The expedition formally began on 16 September 2011, with the departure from the ISS of the Soyuz TMA-21 spacecraft. Astronauts Satoshi Furukawa, Michael Fossum and Sergey Volkov, who had arrived at the ISS aboard Soyuz TMA-02M in June 2011, began their Expedition 29 service at this time.  - Expedition 27 was the 27th long-duration expedition to the International Space Station (ISS), starting on 16 March 2011. Expedition 27 saw numerous notable events, including the undocking of the Progress M-09M and Kounotori 2 spacecraft, the arrival of the Soyuz TMA-21 and Progress M-10M spacecraft, and the final rendezvous with the ISS of NASA's Space Shuttle "Endeavour, on its last mission, STS-134. The expedition ended on 23 May 2011 with the departure of the Soyuz TMA-20 spacecraft, although command of the station was ceremonially handed over to the crew of Expedition 28 on 22 May.  - Soyuz TMA - 21 ( `` Gagarin '' ) was a Soyuz flight to the International Space Station ( ISS ) . It transported three members of the Expedition 27 crew to the ISS , and docked at the station on April 6 , 2011 . TMA - 21 is the 109th flight of a Soyuz spacecraft , the first of which launched in 1967 . The Soyuz remained attached to the space station as a lifeboat , throughout the remainder of Expedition 27 and through the end of Expedition 28 , and returned to Earth on September 16 , 2011 . The launch of Soyuz TMA - 21 was devoted to the 50th anniversary of the first manned space mission , which was conducted by Yuri Gagarin on April 12 , 1961 . The COSPAR International ID of Soyuz TMA - 21 is 2011 - 012A .    What is the relationship between 'soyuz tma-21' and '16 september 2011'?
Answer:
time of spacecraft landing