The adoption of this calendar in some countries was largely due to World War I (Ref: http://europeanhistory.about.com/library/weekly/aa041301c.htm), so in some sense, Howard's birthday depended on that conflict. Which probably explains Schlock Mercenary. So next time, lift a cup to World War I for bringing us this fine webcomic (http://www.schlockmercenary.com/)
Oh - Britain and the US adopted the Gregorian calendar in 1752, jumping from Sept. 2 to Sept. 14. So an amusing bit could be having your intrepid timetravellers try to jump to Sept. 9, 1752, and having the AI refuse :-) Your time-gap may vary, depending on your country?
Happy Birthday, Howard!