Table:
text: 2009 British GT season
Calendar
Calendar
Round Circuit Date_ Pole_Position Fastest_Lap Winning_Drivers Winning_Team
12 Silverstone August 16 Duncan Cameron Matt Griffin Hector Lester Alan Simonsen Duncan Cameron Matt Griffin MTECH Racing
13 Brands Hatch September 19 Andrew Howard Jamie Smyth Alex Mortimer Philip Walker David Jones Godfrey Jones Team Preci-Spark

Question: On how many different dates did the race at Silverstone circuit take place for the 2009 British GT season?