Russia is a big one too, especially for early 20th century history nerds. They used the Julian calendar until after the Revolution, so when reading accounts of

Russia is a big one too, especially for early 20th century history nerds. They used the Julian calendar until after the Revolution, so when reading accounts of, say, most of WW1, Russian sources have dates that are 13 days off from Western sources. This leads to much confusion when authors don’t make it clear they’ve converted the dates (or not). Amusingly, one of the first casualties of this was the October Revolution, which in the new (Gregorian) calendar actually occurred on November 7th