Very good question Phil. I think I'm right in saying that 5 November was the actual date in the Julian Calendar, and so was a date that the plotters would have recognised.
The calendar was changed to the Gregorian version in Britain in 1752, so in that year, we famously lost the days as a result of that.
There's a good synopsis of what happened at the National Maritime Museum website here http://www.nmm.ac.uk/explore/astronomy- ... leap-years
On your last point regarding international differences (the Gregorian calendar was adopted in Europe in 1582), there are conversion tables available to tally up the different dates and I'd venture that historians would tend to talk about the calendar in use in the country they are mainly talking about, or based in.