Something to keep in mind when doing research is that not every country used the same system of dating. In England, dates on official documents, at least those from the fifteenth and sixteenth centuries, tend to be recorded in terms of the monarch’s reign. Those years start with the day the king or queen ascended the throne. Thus, “ii Elizabeth” is the second year of the reign of Elizabeth I of England, which began on November 17, 1559 and ended November 16, 1560.
The Julian calendar was used to date everything else, as it was all over Europe until 1582. In that year, in order to ensure that church holidays occurred in the proper season, Pope Gregory XIII issued a decree that dropped ten days from the calendar Protestant countries, including England and all her colonies, naturally ignored this new Gregorian calendar. Thus, when you read reports about the Spanish Armada in 1588, those written by English witnesses are dated ten days earlier than those written by Spaniards, and the events described take place on different days of the week.
To add to the confusion, in the Elizabethan era the new year was still considered to begin on Lady Day, March 25, even though January 1 was already called New Year’s Day. This is why you will see double dating (February 2, 1588/9) for the early part of some years. The English continued to use the old Julian calendar until 1752, when the date jumped from September 2 to September 14, dropping twelve days to bring the date into alignment with the Gregorian calendar.
For a character who doesn’t read or write and has little idea what a calendar is, and even for one who does, years may also be distinguished in terms of momentous events. Earthquakes, comets, battles—all can be used to mark the passage of time. An Elizabethan speaking in 1590 might well refer to something happening “in the Armada year” rather than say 1588. Be wary, though—this method of dating can only be used after the event!
If you are using a calendar unfamiliar to most of your readers, it may be a good idea to include an author’s note. Chinese, Roman, Jewish, Islamic, and Aztec calendars, even the Republican Calendar used in France from 1793 to 1809, all take some explaining.
Is it necessary to have a dateline? Of course not. In some cases it may be to your advantage not to be too specific. But whether you choose to share the exact date with your readers or not, you will need to create some kind of time line for your own use. If you map out the plot of your story in terms of days of the week, it will help you to avoid contradicting yourself as you write.
[This is an excerpt from How To Write Killer Historical Mysteries: The Art and Adventure of Sleuthing Through the Past (Perseverance Press, 2008) by Kathy Lynn Emerson (aka Kaitlyn Dunnett).]