旧式与新式日期

舊曆（英語：Old Style、缩写：O.S.）与新曆（英語：New Style、缩写：N.S.）分别代表西方國家更換曆法前後的兩套不同日期系统，以表示1582年至20世紀期間各國從儒略曆過渡到格里曆時出現的日期差異。

英格蘭、威爾斯、愛爾蘭和英屬北美於1752年曾變曆兩次；首次将新一年的开始时间从淑女节（英语：Lady Day）（3月25日）调整为1月1日（苏格兰从1600年开始执行），而第二次则於9月的日历中删除11天，以徹底廢儒略历並改行格里历^[1][2]。为了适应此两次變曆，作家们使用双重日期（英语：Dual dating），根据两种日期风格来确定某一天的日期。

对于俄罗斯一類無调整日期變曆的国家，O.S.和N.S.只是表示儒略曆和格里曆的曆法系统，许多东正教国家的教會出于宗教目的，甚至持續沿用儒略历至今。

参考文献

1. Poole 1995，第95–139頁. sfn error: no target: CITEREFPoole1995 (help)
2. Spathaky, Mike Old Style and New Style Dates and the change to the Gregorian Calendar （页面存档备份，存于互联网档案馆）. "Before 1752, parish registers, in addition to a new year heading after 24th March showing, for example '1733', had another heading at the end of the following December indicating '1733/4'. This showed where the Historical Year 1734 started even though the Civil Year 1733 continued until 24th March. ... We as historians have no excuse for creating ambiguity and must keep to the notation described above in one of its forms. It is no good writing simply 20th January 1745, for a reader is left wondering whether we have used the Civil or the Historical Year. The date should either be written 20th January 1745 OS (if indeed it was Old Style) or as 20th January 1745/6. The hyphen (1745-6) is best avoided as it can be interpreted as indicating a period of time."