well, the album that 21 Guns is in - 21st Century Breakdown - was released last May 2009. first single was Know Your Enemy, and then 21 Guns. i dunno though when exactly the singles were released...
i've got all of Green Day's albums, and it says right here in my 21st CB cd that it was released in 2009
It was during Transformer 2
it just came out this year
^^ no it came out in 2010
2009 i meant