Trivia: We used Julian calendar dating on one of our maintenance systems in the US military. Everyone had to learn this dating, and yes it was quite difficult. I asked and never got a good answer on why it was done this way ("Because it's always been this way.")

It's been so long ago, I don't remember too many details, but someone else may have a better memory and pipe up.

More info:

https://en.wikipedia.org/wiki/Gregorian_calendar

https://en.wikipedia.org/wiki/Julian_calendar