It's not Tsar, or Tzar, or Czar. (Which I have all seen used fairly often. Of course, the book I'm using now spells Genghis "Chingis" for some reason) It's Caesar. Those titles are Russian bastardizations of the original Roman title from when they were busy copying the heck out of the West.
OT: Yes, I knew. Yes, I will continue to use AD in everything except my history class. No, I'm not a big fan of bagels.
I find it interesting that one is more common in some places and one in others. Places that are less Christian/more amenable to change use BCE/CE, maybe? BC/AD is predominant in the US, at least where I am, but the US has never liked to change conventions. Standard/Metric system, anyone?