I found the following information incredibly helpful in understanding why some dates appear as they do.  The source of this is “The Researcher’s Guide to American Genealogy” 2nd Edition, Val D. Greenwood, Genealogical Publishing Co., Baltimore, MD, 1993, p41-43

“Calendar difficulties may come as a surprise to you unless you have either studied astronomy or have a good background in history.  However, the calendar and its transition from the Julian to the Gregorian system and other changes involved therewith have considerable impact on many early American genealogical problems.

The main problem has to do with the changing of the calendars – when the switch was made from the Julian to the Gregorian.  In Britain and her colonies (which included most colonies in America) this took place in 1752.  (The Dutch in New Netherland never used the Julian Calendar.  The Dutch had accepted the Gregorian Calendar prior to their American colonization.  These people even continued to use New Style dates in their private records after England had control of their colony.  The Quakers did not accept the ecclesiastical calendar but began their year on January 1 even though they otherwise accepted the dates of the Julian Calendar.)  Remember that 1752 date; it is important.  During the period while the Julian Calendar was used, the Christian church and the countries within which that church prospered used what we call an ecclesiastical calendar which had New Year’s Day falling on March 25.  This was the day of the Feast of the Annunciation which commemorates the visit of the Angel Gabriel to the Virgin Mary to inform her that she would be the mother of the Messiah.  Note that this date is exactly nine months before Christmas, when we celebrate that birth.

Let’s take an example to show the effects of this situation.  You have several documents (such as wills) recorded in chronological order.  The dates on these might run something like this:

November 14, 1718

December 26, 1718

January 3, 1718

January 22, 1718

February 16, 1718

March 5, 1718

March 23, 1718

March 28, 1719

April 12, 1719

This is very simple, isn’t it?  The main difficulty here is that we are accustomed to beginning our years on January 1, so when we see a date like one of these (say February 16, 1718) we automatically put it in the wrong year – we are automatically one year off.

One year off isn’t bad, you say?  This is true unless it leads you to make incorrect conclusions.  If the record in question happens to be a church register and the christenings, etc. of your ancestor’s children recorded therein, you may have a problem.   Let’s say you find two christenings on the following dates for persons you suppose are your ancestor’s children:

April 1, 1720

March 22, 1720

If you didn’t know that the year 1721 began three days after the second of these two christenings, what would be your conclusion?

Or, what about the case of the man who draws his will In October of 1692 and that will is admitted to probate in February 1692?  What would you think?

Because of this problem, we use what we call double-dating.  This means that whenever a date falls between January 1 and March 24, inclusive, before 1752, it should be recorded to reflect both the ecclesiastical and the historical calendars.

Some Christian countries were using the new system of double-dating as early as 1582.

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Many thanks to Debbie McArdle for passing this information on to me!