Medieval Calendar
The Julian calendar (introduced by Julius Caesar in 45 BC) was used throughout Europe until 1582. A standard year had 365 days, and every fourth year (a 'leap year') had an extra day, in order to match the calendar year to the solar year (i.e. the time taken for the earth to orbit the sun, calculated by classical astronomers as 365 ¼ days). This method of fixing the date was known as 'Old Style'. In medieval Europe, including Scotland, the beginning of the year was usually 25 March (the feast of the Annunciation), so that the day after 24 March 1490 was 25 March 1491.

Gregorian Calendar
Medieval scholars noted that the year of 365 ¼ days was a slight overestimate, and by the sixteenth century this had caused a discrepancy of 10 days. Pope Gregory XIII corrected the error by cutting 10 days from the calendar in 1582 (so that 15 October 1582 followed 4 October 1582), and reformed the calendar to make the end year of every fourth century a leap year. The calendar became known as the 'Gregorian Calendar' after Pope Gregory, and dates calculated by the Gregorian Calendar are described as New Style. In addition he decreed that the year should begin on 1 January. However, Pope Gregory's reformation of the calendar was not accepted by most protestant states until the eighteenth century (and the twentieth century in Russia, the Balkans and Greece). Scotland adopted the change to the start of the year in 1599 (31 December 1599 was followed by 1 January 1600). In the rest of the British Isles the change did not take place until 1 January 1752, under Chesterfield's Act (24 Geo. II, c.23), which also removed the eleven days required to bring the British Isles (including Scotland) into New Style (2 September 1752 was followed by 14 September 1752). Some correspondents in the seventeenth and eighteenth centuries would actually mark two dates on a letter, e.g. '17/29 March 1642', to take account of the different calendars operating in different parts of Europe.

Problems for the historian
The historian may have problems correlating dates from different countries: e.g. a letter written in Scotland in February 1625 could be received in England in February 1624. In Scotland it is the custom among archivists and historians to use double dating for dates including the months of January - March prior to 1600, e.g. documents dated 10 March 1586 and 27 March 1587 should be written '10 March 1586/87' and '27 March 1587', respectively.

The calendar in use in Scotland can be summarised as follows:

45BC - October 1582 AD Julian Calendar. Beginning of year was usually 25 March

October 1582AD - 1599 AD Julian Calendar. Beginning of year was usually 25 March
(Some parts of Europe, but not Scotland, now used Gregorian calendar and 1 January as beginning of year).

1600AD - September 1752 AD Julian: but beginning of the year was 1 January
(25 March was still the beginning of the year in England and some other parts of Europe)

September 1752 AD - present Gregorian calendar. Beginning of the year is 1 January for all the British Isles.

For further details on the calendar see to C. R. Cheney (ed.) Handbook of Dates for Students of English History (London, 1978), pp. 1-11; and David Ewing Duncan, The Calendar (1998).

Another problem facing historians of the later middle ages and early modern periods in Scotland, is that clerks sometimes employed a way of writing dates which look distinctly odd to the modern reader. This is the form of dating which, instead of using Arabic numerals (e.g. '23rd June 1632'), used a corrupt Latin form (e.g. 23rd June JajvjC† and threttie twa yeiris). This kind of date, as illustrated in image 2, looks odd to us because they are a mixture of bad Latin and longhand numbers in Scots.

Image 2

An example of a Jaj date ('Jajvijc† and eight yeares'): 1708.

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Image 1

A double date from a document written in Campveere in the Netherlands and sent to Scotland on 21st February 1651 (according to the Gregorian Calendar operating at Campvere) but this was the 11th February 1651 (according to the Julian calendar operating in Scotland at that time). From a document in the National Archives of Scotland (reference: GD40/2/16).