Old Style dates

The adoption of the Gregorian Calendar was an event in the modern history of most nations and societies, marking a change from their traditional (or old style) dating system to the modern (or new style) dating system that is widely used around the world today. Some countries adopted the new calendar from 1582, some did not do so before the early twentieth century, and others did so at various dates between; however a number continue to use a different civil calendar. For many the new style calendar is only used for civil purposes and the old style calendar remains used in religious contexts. Today, the Gregorian calendar is the world's most widely used civil calendar. During – and for some time after – the change between systems, it has been common to use the terms Old Style and New Style when giving dates, to indicate which calendar was used to reckon them.

The Gregorian calendar was decreed in 1582 by the papal bull Inter gravissimas by Pope Gregory XIII, to correct a divergence in the canonical date of the [northern] spring equinox from observed reality (due to an error in the Julian system) that affected the calculation of the date of Easter. Although Gregory's reform was enacted in the most solemn of forms available to the Church, the bull had no authority beyond the Catholic Church and the Papal States. The changes he was proposing were changes to the civil calendar, over which he had no formal authority. They required adoption by the civil authorities in each country to have legal effect.

The bull became the canon law of the Catholic Church in 1582, but it was not recognised by Protestant churches, Orthodox churches, and a few others. Consequently, the days on which Easter and related holidays were celebrated by different Christian churches again diverged.

When converting a date that occurs in a leap year on one calendar but not the other, include 29 February in the calculation if the conversion spans the February/March month change (28 February – 1 March). See table (right).

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