Se denomina calendario gregoriano al calendario promulgado por el Papa Gregorio XIII en 1582 y adoptado inmediatamente por los estados católicos europeos: España (y sus posesiones), Francia, los principados italianos, Portugal, Polonia y los principados católicos del Sacro Imperio. A lo largo de los siglos siguientes fue siendo aceptado por el resto de los países occidentales y, en el siglo XX, por todos los países del mundo. El cambio del calendario juliano o antiguo al calendario gregoriano o moderno no tuvo lugar a la vez en todo el mundo, lo que causa una cierta confusión en la armonización de fechas y en el datado de eventos entre los siglos XVI y XX.

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Old Style (O.S.) and New Style (N.S.) are sometimes used with dates to indicate whether the Julian year has been adjusted to start on 1 January (N.S.), even though documents written at the time use a different start of year (O.S.), or whether a date conforms to the Julian calendar (O.S.), formerly in use in many countries, rather than the Gregorian (N.S.). Closely related is double dating, which uses two consecutive years because of differences in the starting date of the year, or includes both the Julian and Gregorian dates. Beginning in 1582, the Gregorian calendar replaced the Julian in Catholic countries. This change was also implemented in Protestant and Orthodox countries some time later. In England and Wales, Ireland, and the British colonies, the change of the start of the year and the changeover from the Julian calendar occurred in 1752 under the Calendar (New Style) Act 1750. In Scotland, the legal start of the year had already been moved to 1 January (in 1600), but Scotland otherwise continued to use the Julian calendar until 1752. Many cultures and countries now using the Gregorian calendar have different old styles of dating, depending on the type of calendar they used before the change. The Latin equivalents, which are used in many languages, are stili veteris (genitive) or stilo vetere (ablative), abbreviated st.v. and respectively meaning "(of) old style" and "(in) old style", and stili novi or stilo novo, abbreviated st.n. and meaning "(of/in) new style". The Latin abbreviations may be capitalised differently by different users, e.g., St.n. or St.N. for stili novi. There are equivalents for these terms in other languages as well, such as the German a.St. ("alten Stils" for O.S.).

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