I will pre-emptively explain the joke for those who are not from the US
December 5th was the Pearl Harbor Attack
September 11th was the World Trade Center Attack
and April 12th is the day that the American Civil War officially began.
The Pilotside Chronicle
Unapologetically Queer and Queerly Unapologetic
So are those dates Julian or proleptic Gregorian? 😛
I didn’t do that much research into it but found out Greece was the last western country to adopt the Gregorian Calendar (in the 1920s!)