So many things are taken for granted today, and they often slip unnoticed into our interpretation of history. One example is our notions of date and year. Even if we are aware, however dimly, of a change in (Western) calendars from an "Old Style" or Julian to a "New Style" or Gregorian system, this seems something remote from our "living" of our past and interpreting that history. But of such things are the authenticity and accuracy of this endeavor made.
There are a number of books and online resources ( http://www.earlymodernweb.org.uk/emn/index.php/archives/2004/12/calendars-and-dates/ is just one example; you can Google others) that detail the history of this calendar conundrum, so I won't rehearse the "story" here. However, let me sketch out a "thumbnail" of the matter and how it might affect our "living history" interpretation. (I'm not going to "touch" the French calendar of the Revolution; suffice it to say, the issue of dates, years, and calendars can easily spiral out of control.)
Due to the fact that the "old" Roman, Julian (after Julius Caesar), calendar was becoming increasingly out of synch with the round of seasons (said calendar not accounting for the fact that one circuit around the Sun took 365.2422 days), a new calendar was devised and promulgated in 1582 by Pope Gregory XIII (Gregorian) .
Sounds so simple. A calendar of 365 days, with an extra day every four years (in years dividable by 4) to account for that nettlesome fraction. If only.
Depending upon where you lived, this new calendar was or wasn't "in play" until:
1582 France, Portugal, Spain
1700 Denmark, Hamburg
1752 Great Britain, Ireland
What practical difference did it make? Well, if you were trading or traveling between any of these countries or their colonies, you would need to account for the difference (time traveling was a frequent practice; "in the Glorious Revolution of William of Orange[left] Holland on 11 November 1688 New Style and [reached] England on 5 November 1888 Old Style."*) There was a difference of 10 days between the two calendars until 17 February 1800 when the difference became 11 days.
Figuring sailing time, order of transactions, exchange rates, birth/death dates (unless noted) and more all hinge on knowing what date and "who's" date. As if the French and English didn't have enough to argue over.
But wait! There's more.
Up until the the adoption of the Gregorian calendar, the New Year was considered to be 25 March, under the Julian calendar--the first day of the new year. Only after the adoption of the Gregorian calendar did this become 1 January.
What's the practical upshot? Beginning in the last quarter of the 17th century, the English and others still using the Julian calendar began to refer to the days between 1 January and 25 March by BOTH the old and new year (ie 1698/99, 1715/16, and so on). This practice wasn't widespread until the 18th century.
So, documents from this time period will note the year thusly, and the date is also often noted as OS (Old Style) or NS (New Style) at other times. A document from this time period with "11.11.11", say in Philadelphia, would indicate 11 January 1711/12.
Check your papers now, so you really know what time it is.