It's not as simple as just plugging in the numbers after January 20, 2008 and considering everything after that part of the deficit President Obama ran up. Generally, there are a lot of spending mandates left over from the previous President's term that in a political analysis, should be attributed to Bush and not to Obama. The cost of the tax cuts, run over the course of four years, should be added to Bush's deficit total, since that was a Bush administration initiative. The costs of wars in Afghanistan and Iraq should be considered Bush spending. The interest on the debt run up during the Reagan, Bush I and Bush II administrations should not be attributed to the current President.
Obviously, accounting simply takes into consideration the fiscal year, but politically, attributing spending to one President that was mandated by another is not accurate. No matter what position you've taken, there are things from previous administrations that the sitting President is stuck with.
Say what you want, President Obama did not waste a trillion dollars, and five thousand American lives, on a war that was based on false pretenses and accomplished nothing related to either its stated objective, or to enhance the position of America's foreign policy in the Middle East. Don't talk about Benghazi until Iraq has been fully investigated and those responsible for that debacle are held accountable.