When it comes to historical events that happened centuries ago, it's easy to get dates mixed up because those time periods weren't exactly relevant to you. Nor did you live through them.
However, if you're mixing up major historical events that had occurred in the last few, let's say, Presidential terms, there's really no excuse to confuse them.
In the case of some die-hard Obama haters, people pulling at straws to criticize other politicians so the people they support don't look so gosh-darn awful by comparison, also known as a straw-man argument. However, this particular straw-man had some overzealous conservatives respond to criticism of President Trump's response to Hurricane Harvey by claiming Obama was playing golf while in office during Hurricane Katrina.