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Double-dating: Treaties under the Julian and Gregorian Calendars

By: Randall Lesaffer

Anyone browsing through Oxford Historical Treaties will almost immediately notice that many of the treaties are identified with two different dates. This phenomenon can be observed early in the series with the peace treaties of Osnabrück (1 CTS 119) and Münster (1 CTS 271), both of which are dated 14 (24) October 1648. This double-dating is consequential to the different times at which European powers moved from the Julian calendar to the Gregorian calendar. The latter was promulgated by Pope Gregory XIII (1572–1585) in the bull Inter gravissimas of 24 February 1582.

The date of identification used for each treaty in the Consolidated Treaty Series (and, by extension, in Oxford Historical Treaties) is the date of the signature of the treaty by its negotiators, except where it is otherwise indicated. This is the customary way to date treaties, and the most practical one, but it is only one of several possible options. The vast majority of the treaties from the period after 1648 were made through a multi-phase procedure whereby the treaty text was negotiated, drafted, finalised, and signed by representatives of the principal signatories, and subsequently ratified by those principals. This procedure, the forms of which had become largely crystallised by the 12th–13th centuries, had almost completely superseded the older form whereby the principals agreed to the treaty in one another’s presence (which had been dominant through much of the Middle Ages).

The multi-phase procedure involved the production of different sets of legal instruments. The first was the text of the agreement signed by the diplomats. The second were the separate documents which attested the ratification by each of the actual treaty parties – the sovereigns – and which were signed and sealed by them. Treaties normally provided a term within which the signatories would ratify the treaty, which could even vary per party; they did not impose an exact date. By consequence, the ratification by the treaty parties occurred at different moments. Although treaties as a rule only became binding upon ratification by all parties, it is thus more practical to date treaties from the date of signature of the negotiation instrument as this is frequently the only document which is common to all parties. A third option for dating treaties is their date of publication. Again, this can differ per party and even per region. A fourth would be the date of their entry into force, but even this could differ, mostly per region. A fifth could be the date of exchange of ratification documents, but often these were just turned over at different moments rather than being simultaneously exchanged.

Before the introduction of the Gregorian calendar in 1582, Christian Europe lived by the Julian calendar of 46 BC. Nevertheless, some differences in dating did exist under the Julian calendar, for various reasons, including the dates on which the new year began in different countries. Whereas the new year began on 1 January in most of Christian Europe – according to Catholic computation the day of the circumcision of Christ – England until 1752 adhered to the style of the Annunciation, with the new year beginning on 25 March. For example, when the date was 24 March 1700 in Sweden, it would have been 24 March 1699 in England (though the next day in England would have been 25 March 1700).The Consolidated Treaty Series did not specifically address the resulting discrepancies in the different calendars but chose instead to indicate all years according to the continental style (i.e., considering the new year to have begun on 1 January). For example, the Consolidated Treaty Series gives a date of 8 (18) March 1675 for the Treaty of London between England and the Dutch Republic, although in fact the dates of signature were 8 March 1674 for the English and 18 March 1675 for the Dutch. In the treaty texts themselves, this problem is often dealt with by expressly indicating at the inception that the date is in the ‘old style’ for the English custom; afterwards the year is not repeated but the treaty generically mentions a date, for instance, as ‘this 8 March’ (Treaty of London of 8 (18) March 1675, 13 CTS 355).

The reform of the calendar of 1582 was deemed necessary because the calendar had become disjointed from the solar year. The Gregorian calendar reform entailed two major changes. Firstly, it was decided that 4 October 1582 would be followed by 15 October 1582, so as to bring the calendar in line with the solar year. Secondly, it was decided that all years divisible by 100 but not by 400 – i.e., 1700, 1800, and 1900 but not 1600 or 2000 – would be common years of 365 days rather than leap years of 366 days.

Spain, Portugal, and the Italian states introduced the Gregorian calendar at the stated time and skipped 10 days between 4 and 15 October 1585. France followed before the end of the year (10–19 December 1582), with the Catholic part of the Holy Roman Empire adhering in 1583. But the Orthodox countries and the vast majority of the Protestant countries – with the exception of most of the Dutch Republic, which joined at the end of 1582 – continued to use the Julian calendar. As a consequence of this, the calendar of the Catholic world had jumped 10 days ahead of that of the Orthodox and Protestant worlds. After the omission of a leap year in 1700, this gap grew to 11 days for the 18th century, further widening to 12 days in the 19th century and 13 days in the 20th century. This gives the following scheme:

Gregorian calendar date between 15 October 1582 and 28 February 1700 = Julian calendar date + 10 days
Gregorian calendar date between 1 March 1700 and 28 February 1800 = Julian calendar date + 11 days
Gregorian calendar date between 1 March 1800 and 28 February 1900 = Julian calendar date + 12 days
Gregorian calendar date between 1 March 1900 and 28 February 2100 = Julian calendar date + 13 days

The Gregorian calendar became known as the ‘new style’, whereas the Julian was referred to as the ‘old style’. Confusingly, as already indicated above, the same distinguishing terminology was also used to refer to the continental (new) and English style of the new year.

For most of the treaties in Oxford Historical Treaties, one cannot simply apply a Catholic v. Protestant division. The picture is blurred by the gradual adoption of the Gregorian calendar by Protestant powers. For example, the Duchy of Prussia adopted the Catholic calendar relatively quickly (23 August–1 September 1610). The Protestant estates of the Holy Roman Empire as well as Denmark and the Protestant cantons of the Swiss Confederation made the leap between 19 and 29 February 1700. As mentioned above, England and its empire did not take that step until 1752 (3–13 September 1752), at which time the new year was also moved to 1 January. Sweden followed a year later (16–28 February 1753).

The Orthodox world only followed at the beginning of the 20th century. By consequence, in most of the treaties in Oxford Historical Treaties, references to old-style dates are included for treaties involving Orthodox powers. Russia moved to the Gregorian calendar after the Russian Revolution (1–14 February 1918). It had been preceded in 1916 by Bulgaria (1–14 April 1916). Serbia waited until 1919 (14–27 January 1919), Greece until 1923 (16 February–1 March 1923), and Romania until 1924 (1–14 November 1924). In most of these cases, the reform was imposed by civil authority, with the Orthodox Church reluctantly following.

Comment on Notation in Oxford Historical Treaties

In the case of treaties that included a party that had not yet moved from the Julian calendar to the Gregorian calendar, the Oxford Historical Treaties uses the notation shown above, with the Julian date followed by the Gregorian date (i.e., the later of the two dates) in brackets/parentheses; a space does not usually appear before the parenthetical unless the parenthetical (Gregorian) date is in a different month. However, researchers wishing to use the “Advanced search” feature in Oxford Historical Treaties in order to search for an exact date must use the Gregorian (later) date in order to find a treaty wherever this ‘double-dating’ notation is used; although the Julian dates are shown on the main treaty record, only Gregorian dates are recorded in the “date signed” fields that are used to perform such searches.


Roger Beckwith, Calendar and Chronology, Jewish and Christian: Biblical, Intertestamental and Patristic Studies (Leiden: Brill, 1996).

Bonnie Blackburn and Leofranc Holford-Strevens, The Oxford Companion to the Year (Oxford University Press, 2003).

Ludwig Bittner, Die Lehre von den völkerrechtlichen Vertragsurkunden (Stuttgart, Berlin and Leipzig: Deutsche Verlags-Anstalt, 1924).

Randall Lesaffer, ‘Peace Treaties from Lodi to Westphalia’, in Randall Lesaffer (ed.), Peace Treaties and International Law in European History: From the End of the Middle Ages to World War One (Cambridge University Press, 2004) 9-46, at 22-9.

Agnes Kirsopp Michels, The Calendar of the Roman Republic (Princeton University Press, 1967).

Reginald Lane Poole, Medieval Reckonings of Time, Helps from Students of History Vol. III (London: Society for Promoting Christian Knowledge, 1921).

E.G. Richards, Mapping Time. The Calendar and Its History (Oxford University Press, 1998).

Anna-Dorothee von den Brincken, Historische Chronologie des Abendlandes. Kalenderreformen und Jahrtausendrechnungen (Stuttgart, Berlin and Köln: Verlag W. Kohlhammer, 2000).