The way we recognize the passage of time is an interesting study in and of itself. Over the centuries, we've slowly shifted toward a more unified system for determining the months and days, though there are still some old traditions and religions to keep different systems alive. Today I'd like to go over an interesting element concerning an issue that may arise with reviewing historical documents, most notably those of English origin.
When the Gregorian Calendar was adopted in the late 16th Century, a lot of countries, including England, refused to "adapt" for quite a while, and those who had changed over called us fools for sticking with the less precise calendar we'd always used. Part of it was a political game, as Protestant countries didn't want to follow the new Catholic Calendar, but that's a whole other story.
Doing genealogical research, the disparity of the two calendars becomes quite apparent, with the "dual date months" that start to appear. When you run across birthdays, anniversaries, etc... that take place during January, February, and March in years prior to 1752 in the English speaking world, you have to keep in mind that the year of the event is actually incorrect, for they continued to recognize the first day of the year as March 25th. Say you have an ancestor born February 5, 1690 in Salem, Massachusetts. If they were truly born in 1690 by our current standard, the birth record would list their birth as 1689, because the "New Year" wouldn't have happened yet by their record. This is something important to remember if you ever have access to a time machine. You might show up a year too late for an event!
Of course, if you run into Russian dates, it's even worse. They didn't switch to the Gregorian calendar until the Communists took over. Imagine that, it took the Commies to adopt a Catholic creation!
I suppose all these calendar quandaries will be null and void soon. After all, the Mayan calendar is about to end on December 21, so we're all doomed. Doomed! Well, maybe not.